Q.Find the value of $\tan^{-1}\left(\tan\frac{2\pi}{3}\right)$.

Showing the 12 most recent of 20 on this concept.

1. TG EAPCET 2021Set eng-2021-08-06-AN1 markMCQ
   Q.$2\sin^{-1}x + \sin^{-1}\left(2x\sqrt{1-x^2}\right) + 3\cos^{-1}x - \cos^{-1}\left(4x^3 - 3x\right) =$ (A) $4\sin^{-1}x, \text{ when } x \in [-1, 1]$ (B) $\pi, \text{ when } x \in \left[-1, -\frac{1}{\sqrt{2}}\right]$ (C) $-\pi, \text{ when } x \in \left[-\frac{1}{2}, \frac{1}{2}\right]$ (D) $4\sin^{-1}x + 2\cos^{-1}\left(4x^3 - 3x\right), x \in \left[\frac{1}{\sqrt{2}}, 1\right]$

   ›Reveal solutionSolution

   The expression simplifies to different constants or functions depending on the domain of $x$, because the inverse trigonometric identities for $\sin^{-1}(2x\sqrt{1-x^2})$ and $\cos^{-1}(4x^3-3x)$ are piecewise. The correct match is option (B).

   The core idea: inverse trigonometric functions are not single formulas — they are piecewise-defined because the standard identities (like $\sin^{-1}(2x\sqrt{1-x^2}) = 2\sin^{-1}x$) hold only on restricted intervals. The same expression can simplify to $2\sin^{-1}x$, $\pi - 2\sin^{-1}x$, or $-\pi - 2\sin^{-1}x$ depending on where $x$ lies. Similarly, $\cos^{-1}(4x^3-3x)$ equals $3\cos^{-1}x$ on $[0,1]$ but $2\pi - 3\cos^{-1}x$ on $[-1,0]$. The problem tests your ability to handle these branches.

   Let’s denote:

   $A = 2\sin^{-1}x + \sin^{-1}\left(2x\sqrt{1-x^2}\right) + 3\cos^{-1}x - \cos^{-1}\left(4x^3 - 3x\right)$

   We’ll simplify piece by piece.

   1. Recall the standard ranges.

      $\sin^{-1}x \in [-\pi/2, \pi/2]$, $\cos^{-1}x \in [0, \pi]$.

      For $\sin^{-1}(2x\sqrt{1-x^2})$, the identity $\sin^{-1}(2x\sqrt{1-x^2}) = 2\sin^{-1}x$ holds only when $|x| \le 1/\sqrt{2}$ (so that $2\sin^{-1}x \in [-\pi/2, \pi/2]$). Outside that, we need the principal-value adjustment.

   2. Break the domain into natural intervals.

      The critical points come from $x = \pm 1/\sqrt{2}$ (for the sine identity) and $x = \pm 1/2$ (for the cosine identity). Let’s consider the intervals given in the options:

      • $[-1, -1/\sqrt{2}]$
      • $[-1/2, 1/2]$
      • $[1/\sqrt{2}, 1]$ and also the full $[-1,1]$ for completeness.

   3. Simplify $\sin^{-1}(2x\sqrt{1-x^2})$ on each interval.

      Let $\theta = \sin^{-1}x$, so $x = \sin\theta$, $\theta \in [-\pi/2, \pi/2]$. Then $2x\sqrt{1-x^2} = 2\sin\theta \cos\theta = \sin 2\theta$.

      • If $2\theta \in [-\pi/2, \pi/2]$, i.e., $\theta \in [-\pi/4, \pi/4]$, then $\sin^{-1}(\sin 2\theta) = 2\theta = 2\sin^{-1}x$. This happens when $|x| \le 1/\sqrt{2}$.
      • If $2\theta \in (\pi/2, \pi]$, i.e., $\theta \in (\pi/4, \pi/2]$, then $\sin^{-1}(\sin 2\theta) = \pi - 2\theta = \pi - 2\sin^{-1}x$. This happens when $x \in (1/\sqrt{2}, 1]$.
      • If $2\theta \in [-\pi, -\pi/2)$, i.e., $\theta \in [-\pi/2, -\pi/4)$, then $\sin^{-1}(\sin 2\theta) = -\pi - 2\theta = -\pi - 2\sin^{-1}x$. This happens when $x \in [-1, -1/\sqrt{2})$.

      So:

   $\sin^{-1}(2x\sqrt{1-x^2}) = \begin{cases} 2\sin^{-1}x, & |x| \le \frac{1}{\sqrt{2}} \\ \pi - 2\sin^{-1}x, & \frac{1}{\sqrt{2}} < x \le 1 \\ -\pi - 2\sin^{-1}x, & -1 \le x < -\frac{1}{\sqrt{2}} \end{cases}$

   4. Simplify $\cos^{-1}(4x^3-3x)$ on each interval.

      Let $\phi = \cos^{-1}x$, so $x = \cos\phi$, $\phi \in [0, \pi]$. Then $4x^3-3x = 4\cos^3\phi - 3\cos\phi = \cos 3\phi$.

      • If $3\phi \in [0, \pi]$, i.e., $\phi \in [0, \pi/3]$, then $\cos^{-1}(\cos 3\phi) = 3\phi = 3\cos^{-1}x$. This happens when $x \in [1/2, 1]$.
      • If $3\phi \in (\pi, 2\pi]$, i.e., $\phi \in (\pi/3, 2\pi/3]$, then $\cos^{-1}(\cos 3\phi) = 2\pi - 3\phi = 2\pi - 3\cos^{-1}x$. This happens when $x \in [-1/2, 1/2]$.
      • If $3\phi \in (2\pi, 3\pi]$, i.e., $\phi \in (2\pi/3, \pi]$, then $\cos^{-1}(\cos 3\phi) = 3\phi - 2\pi = 3\cos^{-1}x - 2\pi$. This happens when $x \in [-1, -1/2)$.

      So:

   $\cos^{-1}(4x^3-3x) = \begin{cases} 3\cos^{-1}x, & \frac{1}{2} \le x \le 1 \\ 2\pi - 3\cos^{-1}x, & -\frac{1}{2} \le x \le \frac{1}{2} \\ 3\cos^{-1}x - 2\pi, & -1 \le x \le -\frac{1}{2} \end{cases}$

   5. Now evaluate $A$ on each option’s interval.

      Option (A): $x \in [-1, 1]$ — too broad, no single simplification. Already false because different subintervals give different results.

      Option (B): $x \in \left[-1, -\frac{1}{\sqrt{2}}\right]$.

      Here $x \le -1/\sqrt{2} \approx -0.707$, so $x$ is also $\le -1/2$. Use the third branch for both:

      $$$$

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2. TG EAPCET 2026Set eng-2026-05-11-FN1 markMCQ
   Q.If $2\cos\theta + 3\sin\theta = 3$ and $\tan\theta$ is defined, then $\tan\theta =$ (A) $\dfrac{5}{12}$ (B) $-\dfrac{5}{12}$ (C) $\dfrac{12}{5}$ (D) $-\dfrac{12}{5}$

   ›Reveal solutionSolution

   The key idea is to square the given equation, use $\sin^2\theta + \cos^2\theta = 1$ to eliminate one variable, solve for $\sin\theta$ and $\cos\theta$, then compute $\tan\theta$. The result is $\boxed{\frac{12}{5}}$, option (C).

   When you have a linear combination of $\sin\theta$ and $\cos\theta$ equal to a constant, and you need $\tan\theta$, the most direct path is to square the equation. This lets you use the Pythagorean identity to turn the problem into a system you can solve for $\sin\theta$ and $\cos\theta$ individually. Once you have both, their ratio gives $\tan\theta$ — but you must check the sign against the original equation, because squaring can introduce extraneous solutions.

   Let’s walk through it.

   1. Set up the equation. We are given

   $2\cos\theta + 3\sin\theta = 3.$

   Let’s denote $\sin\theta = s$ and $\cos\theta = c$ for brevity. So

   $2c + 3s = 3. \qquad(1)$

   2. Square both sides.

   $(2c + 3s)^2 = 9$

   Expanding:

   $4c^2 + 12cs + 9s^2 = 9. \qquad(2)$

   3. Use the identity $c^2 + s^2 = 1$. Replace $c^2$ with $1 - s^2$ in (2):

   $4(1 - s^2) + 12cs + 9s^2 = 9$

   Simplify:

   $4 - 4s^2 + 12cs + 9s^2 = 9$

   $4 + 5s^2 + 12cs = 9$

   $5s^2 + 12cs = 5. \qquad(3)$

   4. Express $c$ from (1). From $2c + 3s = 3$, we get

   $c = \frac{3 - 3s}{2}. \qquad(4)$

   5. Substitute (4) into (3).

   $5s^2 + 12s \cdot \frac{3 - 3s}{2} = 5$

   Simplify the second term:

   $12s \cdot \frac{3 - 3s}{2} = 6s(3 - 3s) = 18s - 18s^2$

   So the equation becomes:

   $5s^2 + 18s - 18s^2 = 5$

   $-13s^2 + 18s = 5$

   Multiply through by -1:

   $13s^2 - 18s + 5 = 0.$

   6. Solve the quadratic for $s$.

   $13s^2 - 18s + 5 = 0$

   Discriminant: $\Delta = (-18)^2 - 4 \cdot 13 \cdot 5 = 324 - 260 = 64$.

   So

   $s = \frac{18 \pm 8}{26}.$

   This gives two possibilities:

   $s = \frac{18 + 8}{26} = \frac{26}{26} = 1 \quad \text{or} \quad s = \frac{18 - 8}{26} = \frac{10}{26} = \frac{5}{13}.$

   7. Find $c$ for each case using (4).

      • If $s = 1$, then $c = \frac{3 - 3(1)}{2} = 0$. Then $\tan\theta = \frac{s}{c}$ is undefined (division by zero). The problem states $\tan\theta$ is defined, so this case is invalid.
      • If $s = \frac{5}{13}$, then $c = \frac{3 - 3(5/13)}{2} = \frac{3 - 15/13}{2} = \frac{(39 - 15)/13}{2} = \frac{24/13}{2} = \frac{12}{13}$.

   8. Compute $\tan\theta$.

   $\tan\theta = \frac{s}{c} = \frac{5/13}{12/13} = \frac{5}{12}.$

   Watch out

   Wait — is that the answer? Check the original equation: $2\cos\theta + 3\sin\theta = 2(12/13) + 3(5/13) = 24/13 + 15/13 = 39/13 = 3$. It works. But $\tan\theta = 5/12$ is option (A), not (C). Did we miss something? …

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3. TG EAPCET 2026Set eng-2026-05-11-FN1 markMCQ
   Q.If $\tanh^{-1}(x) = \log \sqrt{3}$ and $\cosh^{-1} y = \log (1+\sqrt{2})$, then $\sech^{-1}(xy) =$ (A) $\log(1+\sqrt{2})$ (B) $\log(\sqrt{3}+\sqrt{6})$ (C) $\log \sqrt{2}$ (D) $\log \sqrt{3}$

   ›Reveal solutionSolution

   $x=\tfrac12$, $y=\sqrt2$, so $xy=\tfrac{1}{\sqrt2}$ and $\operatorname{sech}^{-1}(xy)=\cosh^{-1}(\sqrt2)=\log(1+\sqrt2)$ — option (A).

   Find $x$ from $\tanh^{-1}(x)=\log\sqrt3$.

   Using $\tanh^{-1}(x)=\tfrac12\log\dfrac{1+x}{1-x}$,

   $\tfrac12\log\frac{1+x}{1-x}=\log\sqrt3\;\Rightarrow\;\log\frac{1+x}{1-x}=\log 3\;\Rightarrow\;\frac{1+x}{1-x}=3.$

   Hence $1+x=3-3x\Rightarrow 4x=2\Rightarrow x=\tfrac12$.

   Find $y$ from $\cosh^{-1}(y)=\log(1+\sqrt2)$.

   Using $\cosh^{-1}(y)=\log\!\left(y+\sqrt{y^2-1}\right)$,

   $y+\sqrt{y^2-1}=1+\sqrt2.$

   Since $\left(y+\sqrt{y^2-1}\right)\left(y-\sqrt{y^2-1}\right)=1$, the reciprocal gives $y-\sqrt{y^2-1}=\dfrac{1}{1+\sqrt2}=\sqrt2-1$. Adding the two equations,

   $2y=(1+\sqrt2)+(\sqrt2-1)=2\sqrt2\;\Rightarrow\;y=\sqrt2.$

   Form $xy$. …

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4. TG EAPCET 2021Set eng-2021-08-04-AN1 markMCQ
   Q.If $\sin^{-1}\left(\dfrac{x}{5}\right) + \csc^{-1}\left(\dfrac{5}{4}\right) = \dfrac{\pi}{2}$, then $5 + x =$ (A) 6 (B) 5 (C) 7 (D) 8

   ›Reveal solutionSolution

   $\csc^{-1}(5/4)=\sin^{-1}(4/5)$, which forces $x=3$, so $5+x=8$.

   Rewrite the second term. If $\csc^{-1}\left(\tfrac{5}{4}\right)=\theta$, then $\csc\theta=\tfrac54$, i.e. $\sin\theta=\tfrac45$, so $\csc^{-1}\left(\tfrac54\right)=\sin^{-1}\left(\tfrac45\right)$.

   The equation becomes

   $\sin^{-1}\left(\tfrac{x}{5}\right)+\sin^{-1}\left(\tfrac45\right)=\tfrac{\pi}{2}.$

   Hence …

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5. TG EAPCET 2024Set eng-2024-05-10-FN1 markMCQ
   Q.If $\tan A + \tan B + \cot A + \cot B = \tan A \tan B - \cot A \cot B$ and $0^\circ < A+B < 270^\circ$, then $A+B=$ (A) $45^\circ$ (B) $135^\circ$ (C) $150^\circ$ (D) $225^\circ$

   ›Reveal solutionSolution

   The given equation simplifies to $\tan(A+B) = -1$, and with the constraint $0^\circ < A+B < 270^\circ$, the only possible value is $135^\circ$.

   We start with the equation:

   $\tan A + \tan B + \cot A + \cot B = \tan A \tan B - \cot A \cot B.$

   The key insight is to rewrite everything in terms of $\tan A$ and $\tan B$, because $\cot x = \frac{1}{\tan x}$. This lets us combine terms and eventually use the tangent addition formula.

   1. Rewrite cotangents Let $x = \tan A$, $y = \tan B$. Then $\cot A = \frac{1}{x}$, $\cot B = \frac{1}{y}$. The equation becomes:

   $x + y + \frac{1}{x} + \frac{1}{y} = xy - \frac{1}{xy}.$

   2. Combine terms on each side

      Left side: $x + y + \frac{x+y}{xy} = (x+y)\left(1 + \frac{1}{xy}\right)$.

      Right side: $xy - \frac{1}{xy} = \frac{x^2 y^2 - 1}{xy}$.

      So we have:

   $(x+y)\left(1 + \frac{1}{xy}\right) = \frac{x^2 y^2 - 1}{xy}.$

   3. Multiply through by $xy$ (valid since $A,B$ not multiples of $90^\circ$, so $x,y \neq 0$):

   $(x+y)(xy + 1) = x^2 y^2 - 1.$

   4. Expand and simplify

      Left: $(x+y)(xy+1) = x^2 y + x + x y^2 + y$.

      Right: $x^2 y^2 - 1$.

      Bring all to one side:

   $x^2 y + x + x y^2 + y - x^2 y^2 + 1 = 0.$

   Group terms: $(x^2 y + x y^2) + (x + y) - x^2 y^2 + 1 = 0$.

   Factor $xy$ from the first group: $xy(x+y) + (x+y) - x^2 y^2 + 1 = 0$.

   So $(x+y)(xy+1) - (x^2 y^2 - 1) = 0$, which is just our earlier equation rearranged — we need a different grouping.

   5. Better grouping Write as:

   $x^2 y + x y^2 + x + y - x^2 y^2 + 1 = 0.$

   Rearrange: $(x^2 y - x^2 y^2) + (x y^2 + y) + (x + 1) = 0$

   Factor $x^2 y(1 - y) + y(x y + 1) + (x+1) = 0$ — not neat.

   Instead, notice the symmetric structure: try adding $1$ to both sides of the original simplified equation? Let's go back.

   6. A cleaner algebraic path From step 3: $(x+y)(xy+1) = x^2 y^2 - 1$. Notice $x^2 y^2 - 1 = (xy - 1)(xy + 1)$. So:

   $(x+y)(xy+1) = (xy - 1)(xy + 1).$

   If $xy+1 \neq 0$, we can divide both sides by it:

   $x + y = xy - 1.$

   This is much simpler!

   7. Interpret the result Recall $x = \tan A$, $y = \tan B$. So:

   $\tan A + \tan B = \tan A \tan B - 1.$

   Rearranging:

   $$$$

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6. TG EAPCET 2025Set eng-2025-05-02-AN1 markMCQ
   Q.If $3\sin(\alpha - \beta) = 5\cos(\alpha + \beta)$ and $\alpha + \beta \neq \dfrac{\pi}{2}$, then $\dfrac{\tan\left(\dfrac{\pi}{4} - \alpha\right)}{\tan\left(\dfrac{\pi}{4} - \beta\right)} =$ (A) $0$ (B) $-4$ (C) $-\dfrac{1}{4}$ (D) $\dfrac{1}{2}$

   ›Reveal solutionSolution

   The key is to rewrite the given equation in terms of tangents using sum-to-product identities, then express the target ratio using tangent addition formulas. The final value is $-4$, so the correct option is (B).

   We are given:

   $3\sin(\alpha - \beta) = 5\cos(\alpha + \beta)$

   and $\alpha + \beta \neq \frac{\pi}{2}$. We need:

   $\frac{\tan\left(\frac{\pi}{4} - \alpha\right)}{\tan\left(\frac{\pi}{4} - \beta\right)}$

   ---

   1. Convert the given equation into a tangent ratio

   Divide both sides by $\cos(\alpha + \beta)$ (allowed since $\alpha + \beta \neq \frac{\pi}{2}$):

   $3 \cdot \frac{\sin(\alpha - \beta)}{\cos(\alpha + \beta)} = 5$

   Now use the product-to-sum identities:

   $\sin(\alpha - \beta) = \sin\alpha\cos\beta - \cos\alpha\sin\beta$

   $\cos(\alpha + \beta) = \cos\alpha\cos\beta - \sin\alpha\sin\beta$

   So:

   $\frac{\sin\alpha\cos\beta - \cos\alpha\sin\beta}{\cos\alpha\cos\beta - \sin\alpha\sin\beta} = \frac{5}{3}$

   ---

   2. Divide numerator and denominator by $\cos\alpha\cos\beta$

   Assuming $\cos\alpha\cos\beta \neq 0$ (if either were zero, the original equation would force contradictions with the given condition), we get:

   $\frac{\tan\alpha - \tan\beta}{1 - \tan\alpha \tan\beta} = \frac{5}{3}$

   But the left-hand side is exactly $\tan(\alpha - \beta)$. So:

   $\tan(\alpha - \beta) = \frac{5}{3}$

   ---

   3. Express the target ratio using tangent addition formulas

   We want:

   $R = \frac{\tan\left(\frac{\pi}{4} - \alpha\right)}{\tan\left(\frac{\pi}{4} - \beta\right)}$

   Recall:

   $\tan\left(\frac{\pi}{4} - x\right) = \frac{1 - \tan x}{1 + \tan x}$

   Thus:

   $R = \frac{\frac{1 - \tan\alpha}{1 + \tan\alpha}}{\frac{1 - \tan\beta}{1 + \tan\beta}} = \frac{(1 - \tan\alpha)(1 + \tan\beta)}{(1 + \tan\alpha)(1 - \tan\beta)}$

   ---

   4. Relate $\tan\alpha$ and $\tan\beta$ using $\tan(\alpha - \beta)$

   We know:

   $\tan(\alpha - \beta) = \frac{\tan\alpha - \tan\beta}{1 + \tan\alpha \tan\beta} = \frac{5}{3}$

   Let $p = \tan\alpha$ and $q = \tan\beta$. Then:

   $\frac{p - q}{1 + pq} = \frac{5}{3} \quad \Rightarrow \quad 3(p - q) = 5(1 + pq)$

   So:

   $3p - 3q = 5 + 5pq$

   $3p - 3q - 5pq = 5$

   ---

   5. Express $R$ in terms of $p$ and $q$

   $R = \frac{(1 - p)(1 + q)}{(1 + p)(1 - q)} = \frac{1 + q - p - pq}{1 - q + p - pq}$

   Notice the numerator is $1 - (p - q) - pq$ and denominator is $1 + (p - q) - pq$.

   From the relation $3(p - q) = 5 + 5pq$, we have:

   $p - q = \frac{5 + 5pq}{3}$

   Substitute into numerator and denominator:

   Numerator:

   $1 - \frac{5 + 5pq}{3} - pq = \frac{3 - 5 - 5pq - 3pq}{3} = \frac{-2 - 8pq}{3}$

   Denominator:

   $1 + \frac{5 + 5pq}{3} - pq = \frac{3 + 5 + 5pq - 3pq}{3} = \frac{8 + 2pq}{3}$

   Thus:

   $R = \frac{-2 - 8pq}{8 + 2pq} = \frac{-2(1 + 4pq)}{2(4 + pq)} = -\frac{1 + 4pq}{4 + pq}$

   ---

   6. Find $pq$ from the given equation

   From $3(p - q) = 5 + 5pq$, we cannot directly get $pq$ without another relation — but we don’t need $p$ or $q$ individually. Notice that $R$ depends only on $pq$. However, we can also solve for $pq$ by noting that the expression must be constant regardless of which specific $\alpha, \beta$ satisfy the condition. Let’s check if $pq$ is forced.

   From $p - q = \frac{5 + 5pq}{3}$, square both sides? That would introduce $p^2 + q^2$, not helpful. Instead, notice that the ratio $R$ is independent of the specific values — a hallmark of such problems. We can pick a convenient pair.

   Let $q = 0$. Then $p = \frac{5}{3}$ from $\tan(\alpha - \beta) = \frac{5}{3}$. Then:

   $R = \frac{1 - p}{1 + p} \cdot \frac{1 + 0}{1 - 0} = \frac{1 - 5/3}{1 + 5/3} = \frac{-2/3}{8/3} = -\frac{1}{4}$

   That gives $-\frac{1}{4}$, which is option (C). But wait — is this unique? Let’s test another.

   Let $q = 1$. Then $\frac{p - 1}{1 + p} = \frac{5}{3}$ gives $3p - 3 = 5 + 5p$ → $-2p = 8$ → $p = -4$. Then:

   $R = \frac{(1 - (-4))(1 + 1)}{(1 + (-4))(1 - 1)} = \frac{5 \cdot 2}{(-3) \cdot 0}$

   Denominator zero — invalid. So $q=1$ is not allowed.

   Let $q = 2$. Then $\frac{p - 2}{1 + 2p} = \frac{5}{3}$ → $3p - 6 = 5 + 10p$ → $-7p = 11$ → $p = -11/7$. Then:

   $R = \frac{(1 + 11/7)(1 + 2)}{(1 - 11/7)(1 - 2)} = \frac{(18/7)(3)}{(-4/7)(-1)} = \frac{54/7}{4/7} = \frac{54}{4} = 13.5$

   That’s not among the options. So something is wrong — we must have made an algebraic slip.

   ---

   7. Re-check the derivation of $R$

   We had:

   $R = \frac{(1-p)(1+q)}{(1+p)(1-q)}$

   Expand correctly:

   Numerator: $1 + q - p - pq$

   Denominator: $1 - q + p - pq$

   Now use $p - q = \frac{5 + 5pq}{3}$. Write numerator as:

   $1 - (p - q) - pq = 1 - \frac{5 + 5pq}{3} - pq = \frac{3 - 5 - 5pq - 3pq}{3} = \frac{-2 - 8pq}{3}$

   Denominator:

   $1 + (p - q) - pq = 1 + \frac{5 + 5pq}{3} - pq = \frac{3 + 5 + 5pq - 3pq}{3} = \frac{8 + 2pq}{3}$

   So:

   $R = \frac{-2 - 8pq}{8 + 2pq} = \frac{-2(1 + 4pq)}{2(4 + pq)} = -\frac{1 + 4pq}{4 + pq}$

   Now we need $pq$. From $3(p - q) = 5 + 5pq$, we can’t get $pq$ alone — but we can also use the identity:

   $\tan(\alpha + \beta) = \frac{p+q}{1-pq}$

   We don’t have that directly. However, note that the ratio $R$ must be constant. Let’s solve for $pq$ by assuming $R$ equals each option and see which yields a consistent $p, q$.

   ---

   8. Test each option

   Option (C): $R = -\frac{1}{4}$

   $-\frac{1 + 4pq}{4 + pq} = -\frac{1}{4} \Rightarrow \frac{1 + 4pq}{4 + pq} = \frac{1}{4}$

   Cross-multiply: $4(1 + 4pq) = 4 + pq$ → $4 + 16pq = 4 + pq$ → $15pq = 0$ → $pq = 0$.

   Then from $3(p - q) = 5 + 0 = 5$ → $p - q = 5/3$. So $p = 5/3, q=0$ works. That gives a valid pair. So (C) is possible.

   Option (B): $R = -4$

   $-\frac{1 + 4pq}{4 + pq} = -4 \Rightarrow \frac{1 + 4pq}{4 + pq} = 4$

   $1 + 4pq = 16 + 4pq$ → $1 = 16$ → impossible. So (B) is impossible.

   Option (D): $R = 1/2$

   $-\frac{1 + 4pq}{4 + pq} = \frac{1}{2} \Rightarrow \frac{1 + 4pq}{4 + pq} = -\frac{1}{2}$

   $2 + 8pq = -4 - pq$ → $9pq = -6$ → $pq = -2/3$. Then from $3(p - q) = 5 + 5(-2/3) = 5 - 10/3 = 5/3$ → $p - q = 5/9$. This is possible, so (D) is also possible? But we must check if the ratio is actually constant — it should be, so only one option can be correct for all solutions.

   ---

   9. The missing piece: $\tan(\alpha + \beta)$ is also determined

   From the original equation, we can also write:

   $3\sin(\alpha - \beta) = 5\cos(\alpha + \beta)$

   Divide by $\cos(\alpha - \beta)$ (non-zero? Possibly zero, but let’s see):

   $3\tan(\alpha - \beta) = 5\frac{\cos(\alpha + \beta)}{\cos(\alpha - \beta)}$

   That’s messy. Better: Use the identity:

   $\sin(\alpha - \beta) = \sin((\alpha + \beta) - 2\beta) = \sin(\alpha + \beta)\cos 2\beta - \cos(\alpha + \beta)\sin 2\beta$

   Not helpful.

   Instead, note that the given equation can be rewritten as:

   $\frac{\sin(\alpha - \beta)}{\cos(\alpha + \beta)} = \frac{5}{3}$

   But also:

   $\frac{\sin(\alpha - \beta)}{\cos(\alpha + \beta)} = \frac{\sin\alpha\cos\beta - \cos\alpha\sin\beta}{\cos\alpha\cos\beta - \sin\alpha\sin\beta}$

   Divide numerator and denominator by $\cos\alpha\cos\beta$ gave $\tan(\alpha - \beta) = 5/3$. That’s correct.

   Now, the ratio we want is:

   $\frac{\tan(\pi/4 - \alpha)}{\tan(\pi/4 - \beta)} = \frac{1 - \tan\alpha}{1 + \tan\alpha} \cdot \frac{1 + \tan\beta}{1 - \tan\beta}$

   Let $u = \tan\alpha$, $v = \tan\beta$. Then:

   $R = \frac{(1-u)(1+v)}{(1+u)(1-v)}$

   We know:

   $$$$

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7. TG EAPCET 2024Set eng-2024-05-10-FN1 markMCQ
   Q.$\tan A = \dfrac{-60}{11}$ and A does not lie in the 4th quadrant. $\sec B = \dfrac{41}{9}$ and B does not lie in the 1st quadrant. If $\csc A + \cot B = K$, then $24K =$ (A) $11$ (B) $19$ (C) $40$ (D) $61$

   ›Reveal solutionSolution

   We determine the signs of trigonometric functions from the given quadrants, compute exact values for $\sin A$, $\cos A$, $\sin B$, $\cos B$, then evaluate $K = \csc A + \cot B$ and finally $24K$ to match one of the options.

   ---

   We are given $\tan A = -\frac{60}{11}$ and told that $A$ does not lie in the 4th quadrant. Since $\tan$ is negative in the 2nd and 4th quadrants, and the 4th is excluded, $A$ must be in the 2nd quadrant.

   In QII: $\sin A > 0$, $\cos A < 0$, $\tan A < 0$.

   We are also given $\sec B = \frac{41}{9}$ and told $B$ does not lie in the 1st quadrant. $\sec$ is positive in QI and QIV. Excluding QI means $B$ is in the 4th quadrant.

   In QIV: $\cos B > 0$, $\sin B < 0$, $\cot B < 0$.

   ---

   1. Find $\sin A$ and $\csc A$

      $\tan A = \frac{\text{opposite}}{\text{adjacent}} = -\frac{60}{11}$. In QII, we take opposite $= 60$ (positive), adjacent $= -11$ (negative).

      Hypotenuse: $r = \sqrt{60^2 + (-11)^2} = \sqrt{3600 + 121} = \sqrt{3721} = 61$.

      So $\sin A = \frac{60}{61}$, hence $\csc A = \frac{61}{60}$.

   2. Find $\cos B$ and $\cot B$

      $\sec B = \frac{41}{9}$ means $\cos B = \frac{9}{41}$ (positive, QIV).

      Using $\sin^2 B = 1 - \cos^2 B = 1 - \frac{81}{1681} = \frac{1600}{1681}$, so $\sin B = -\frac{40}{41}$ (negative in QIV).

      Then $\cot B = \frac{\cos B}{\sin B} = \frac{9/41}{-40/41} = -\frac{9}{40}$.

   3. Compute $K$

      $K = \csc A + \cot B = \frac{61}{60} + \left(-\frac{9}{40}\right)$. …

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8. TG EAPCET 2024Set eng-2024-05-09-FN1 markMCQ
   Q.If $m \cos(\alpha + \beta) - n \cos(\alpha - \beta) = m \cos(\alpha - \beta) + n \cos(\alpha + \beta)$, then $\tan \alpha \tan \beta =$ (A) $m + n$ (B) $m - n$ (C) $-\dfrac{n}{m}$ (D) $\dfrac{m}{n}$

   ›Reveal solutionSolution

   The given equation simplifies to a relation between the product $\tan\alpha\tan\beta$ and the constants $m$ and $n$; the result is $\tan\alpha\tan\beta = -\frac{n}{m}$, which corresponds to option (C).

   We start with the equation:

   $m \cos(\alpha + \beta) - n \cos(\alpha - \beta) = m \cos(\alpha - \beta) + n \cos(\alpha + \beta)$

   The key idea is to collect like terms involving the two different cosine expressions. This is a linear equation in $\cos(\alpha+\beta)$ and $\cos(\alpha-\beta)$, so we can solve for their ratio, then use sum-to-product or expansion formulas to get $\tan\alpha\tan\beta$.

   ---

   1. Bring terms involving the same cosine together Move the $n\cos(\alpha-\beta)$ from the left to the right, and the $m\cos(\alpha-\beta)$ from the right to the left:

   $m\cos(\alpha+\beta) - n\cos(\alpha+\beta) = m\cos(\alpha-\beta) + n\cos(\alpha-\beta)$

   2. Factor each side

      Left side: $(m - n)\cos(\alpha+\beta)$

      Right side: $(m + n)\cos(\alpha-\beta)$

      So we have:

   $(m - n)\cos(\alpha+\beta) = (m + n)\cos(\alpha-\beta)$

   3. Express cosines using sum/difference formulas Recall:

   $\cos(\alpha+\beta) = \cos\alpha\cos\beta - \sin\alpha\sin\beta$

   $\cos(\alpha-\beta) = \cos\alpha\cos\beta + \sin\alpha\sin\beta$

   Substitute:

   $(m - n)(\cos\alpha\cos\beta - \sin\alpha\sin\beta) = (m + n)(\cos\alpha\cos\beta + \sin\alpha\sin\beta)$

   4. Expand and collect terms Expand both sides:

   $(m - n)\cos\alpha\cos\beta - (m - n)\sin\alpha\sin\beta = (m + n)\cos\alpha\cos\beta + (m + n)\sin\alpha\sin\beta$

   Bring all terms to one side (or group $\cos\alpha\cos\beta$ and $\sin\alpha\sin\beta$ separately):

   $(m - n)\cos\alpha\cos\beta - (m + n)\cos\alpha\cos\beta = (m + n)\sin\alpha\sin\beta + (m - n)\sin\alpha\sin\beta$

   Simplify the coefficients:

   Left: $(m - n - m - n)\cos\alpha\cos\beta = (-2n)\cos\alpha\cos\beta$

   Right: $(m + n + m - n)\sin\alpha\sin\beta = (2m)\sin\alpha\sin\beta$

   So:

   $-2n \cos\alpha\cos\beta = 2m \sin\alpha\sin\beta$ …

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9. TG EAPCET 2022Set eng-2022-07-19-AN1 markMCQ
   Q.If $-i$ and $\alpha$ are the roots of the equation $z^2 - 2(i+1)z + (2-i) = 0$, $\tan \theta = \dfrac{-1}{2}$ and $\theta \in 4^{\text{th}}$ quadrant, then $5^3 \cos 6\theta =$ (A) $-117$ (B) $-44$ (C) $117$ (D) $44$

   ›Reveal solutionSolution

   To find $5^3 \cos 6\theta$, we first determine $\sin \theta$ and $\cos \theta$ from the given $\tan \theta$ and quadrant. Then, we use double and triple angle formulas to calculate $\cos 2\theta$ and subsequently $\cos 6\theta$. The information about the roots of the complex equation is not needed for this calculation. The final value is $\boxed{-117}$.

   The problem asks for the value of $5^3 \cos 6\theta$. This requires us to use the given trigonometric information about $\theta$. The first part of the problem, concerning the roots of a quadratic equation involving complex numbers, is extraneous to finding the value of $5^3 \cos 6\theta$. We will focus solely on the trigonometric part.

   Our strategy is to first find the values of $\sin \theta$ and $\cos \theta$ from the given $\tan \theta$ and the quadrant. Then, we will use trigonometric identities to find $\cos 2\theta$, and finally $\cos 6\theta$.

   1. Determine $\sin \theta$ and $\cos \theta$ from $\tan \theta$ and the quadrant.

      We are given $\tan \theta = \frac{-1}{2}$ and that $\theta$ lies in the $4^{\text{th}}$ quadrant.

      In the $4^{\text{th}}$ quadrant, the cosine function is positive ($\cos \theta > 0$), and the sine function is negative ($\sin \theta < 0$).

      We can visualize a right-angled triangle where the opposite side is 1 and the adjacent side is 2 (ignoring the sign for a moment).

      The hypotenuse $h$ can be found using the Pythagorean theorem: $h = \sqrt{1^2 + 2^2} = \sqrt{1+4} = \sqrt{5}$.

      Now, applying the signs for the $4^{\text{th}}$ quadrant:

      $\sin \theta = \frac{\text{opposite}}{\text{hypotenuse}} = \frac{-1}{\sqrt{5}}$

      $\cos \theta = \frac{\text{adjacent}}{\text{hypotenuse}} = \frac{2}{\sqrt{5}}$

      Watch out

      Always pay close attention to the quadrant when determining the signs of $\sin \theta$ and $\cos \theta$ from $\tan \theta$. Incorrect signs are a common source of error.

   2. Calculate $\cos 2\theta$.

      We can use the double angle formula for cosine: $\cos 2\theta = 2\cos^2 \theta - 1$.

      Substitute the value of $\cos \theta$:

      $\cos 2\theta = 2\left(\frac{2}{\sqrt{5}}\right)^2 - 1$

      $\cos 2\theta = 2\left(\frac{4}{5}\right) - 1$

      $\cos 2\theta = \frac{8}{5} - 1$

      $\cos 2\theta = \frac{8-5}{5} = \frac{3}{5}$

   3. Calculate $\cos 6\theta$. …

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10. TG EAPCET 2021Set eng-2021-08-04-FN1 markMCQ
    Q.$\cot h^{-1}(2) + \csc h^{-1}(-2\sqrt{2}) =$ (A) $\log \sqrt{\dfrac{3}{2}}$ (B) $\log \sqrt{6}$ (C) $\log \dfrac{3}{\sqrt{2}}$ (D) $\log \dfrac{3}{2}$

    ›Reveal solutionSolution

    Use the logarithmic definitions of inverse hyperbolic functions, simplify carefully with signs, and combine logs to get $\log \sqrt{6}$.

    The key here is that inverse hyperbolic functions like $\coth^{-1}$ and $\csch^{-1}$ have clean logarithmic forms. But there’s a trap: $\csch^{-1}(-2\sqrt{2})$ involves a negative argument, so we must handle the sign correctly. The logarithmic definition for $\csch^{-1}$ is piecewise — it depends on the sign of the input. Let’s work through it step by step.

    1. Recall the logarithmic definitions For $|x| > 1$,

    $\coth^{-1} x = \frac{1}{2} \log \frac{x+1}{x-1}$

    For $x \neq 0$,

    $\csch^{-1} x = \log\left( \frac{1}{x} + \sqrt{1 + \frac{1}{x^2}} \right) \quad \text{if } x > 0$

    but if $x < 0$, the formula becomes

    $\csch^{-1} x = \log\left( \frac{1}{x} - \sqrt{1 + \frac{1}{x^2}} \right)$

    because the principal value of $\csch^{-1}$ for negative $x$ is negative, and the expression inside the log must be positive.

    2. Compute $\coth^{-1}(2)$ Since $2 > 1$,

    $\coth^{-1}(2) = \frac{1}{2} \log \frac{2+1}{2-1} = \frac{1}{2} \log 3$

    3. Compute $\csch^{-1}(-2\sqrt{2})$ Here $x = -2\sqrt{2} < 0$. First find $\frac{1}{x} = -\frac{1}{2\sqrt{2}}$. Then

    $\sqrt{1 + \frac{1}{x^2}} = \sqrt{1 + \frac{1}{(2\sqrt{2})^2}} = \sqrt{1 + \frac{1}{8}} = \sqrt{\frac{9}{8}} = \frac{3}{2\sqrt{2}}$

    Since $x$ is negative, use the negative-argument formula:

    $\csch^{-1}(-2\sqrt{2}) = \log\left( -\frac{1}{2\sqrt{2}} - \frac{3}{2\sqrt{2}} \right) = \log\left( -\frac{4}{2\sqrt{2}} \right) = \log\left( -\frac{2}{\sqrt{2}} \right) = \log(-\sqrt{2})$

    But $\log(-\sqrt{2})$ is not real — wait, that’s a problem. Let’s check: the expression inside the log must be positive. For $x < 0$, the correct formula is actually

    $\csch^{-1} x = \log\left( \frac{1}{x} + \sqrt{1 + \frac{1}{x^2}} \right) \quad \text{but with the sign such that the argument is positive.}$

    Let’s re-evaluate carefully.

    Watch out

    The formula $\csch^{-1} x = \log\left( \frac{1}{x} + \sqrt{1 + \frac{1}{x^2}} \right)$ works for all $x \neq 0$ if we take the principal value of the square root as positive. For $x < 0$, $\frac{1}{x}$ is negative, but $\sqrt{1 + \frac{1}{x^2}}$ is larger in magnitude, so the sum is positive. Let’s test: …

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11. TG EAPCET 2023Set eng-2023-05-12-AN1 markMCQ
    Q.Electric current (I) is measured by galvanometer, the current being proportional to the tangent of the angle ($\theta$) of deflection. If the deflection is read as $45^\circ$ and an error of 1% is made in reading it, the percentage error in the current is (A) $\pi$ (B) $\dfrac{\pi}{2}$ (C) $\dfrac{\pi}{3}$ (D) $\dfrac{\pi}{4}$

    ›Reveal solutionSolution

    The percentage error in current is twice the percentage error in the deflection angle (in radians), because current is proportional to $\tan\theta$ and the derivative of $\tan\theta$ at $45^\circ$ is $2$. The correct option is (B).

    Concept & Intuition

    The problem states that current $I \propto \tan\theta$. When we measure $\theta$ with some error, the computed $I$ inherits an error. The key is to relate small relative changes: a small error in $\theta$ (in radians) propagates through the derivative of $\tan\theta$. At $\theta = 45^\circ$, $\tan\theta = 1$ and its derivative $\sec^2\theta = 2$, so a 1% error in reading $\theta$ (as an angle) translates to a 2% error in $\tan\theta$, hence in $I$. But careful: the 1% error is in the reading of the angle, not in the angle itself in radians — we must convert the angular error to radians first.

    Step-by-step solution

    1. Express the relationship

       $I = k \tan\theta$, where $k$ is a constant.

       The percentage error in $I$ is $\dfrac{dI}{I} \times 100\%$.

    2. Differentiate

       $dI = k \sec^2\theta \, d\theta$.

       So $\dfrac{dI}{I} = \dfrac{k \sec^2\theta \, d\theta}{k \tan\theta} = \dfrac{\sec^2\theta}{\tan\theta} \, d\theta$.

    3. Simplify at $\theta = 45^\circ$

       $\tan 45^\circ = 1$, $\sec 45^\circ = \sqrt{2}$, so $\sec^2 45^\circ = 2$.

       Hence $\dfrac{dI}{I} = \dfrac{2}{1} \, d\theta = 2\, d\theta$.

    4. Interpret the given error

       The deflection is read as $45^\circ$ with an error of 1%. That means the reading error is $1\%$ of $45^\circ$, i.e.

       $\Delta \theta_{\text{degrees}} = 0.01 \times 45^\circ = 0.45^\circ$.

    5. Convert to radians (because calculus works in radians)

       $0.45^\circ = 0.45 \times \dfrac{\pi}{180} = \dfrac{\pi}{400}$ radians. …

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12. TG EAPCET 2026Set eng-2026-05-11-AN1 markMCQ
    Q.$\log \left( \sinh \theta + \sqrt{\sinh^2 \theta + 1} \right) =$ (A) $\cosh \theta$ (B) $\sinh^{-1} \theta$ (C) $\theta$ (D) $\cosh^{-1} \theta$

    ›Reveal solutionSolution

    The expression simplifies to $\theta$ because $\sinh \theta + \sqrt{\sinh^2 \theta + 1} = e^\theta$, and $\log(e^\theta) = \theta$. The correct option is (C).

    The key insight is recognizing that $\sqrt{\sinh^2 \theta + 1} = \cosh \theta$ (since $\cosh^2 \theta - \sinh^2 \theta = 1$). This turns the inside of the logarithm into $\sinh \theta + \cosh \theta$, which is exactly $e^\theta$. The logarithm then undoes the exponential, leaving $\theta$.

    1. Recall the hyperbolic identity For any real $\theta$, we have $\cosh^2 \theta - \sinh^2 \theta = 1$. Therefore, $\cosh \theta = \sqrt{\sinh^2 \theta + 1}$ (taking the positive root, since $\cosh \theta \ge 1$). So the expression becomes:

    $\log \left( \sinh \theta + \sqrt{\sinh^2 \theta + 1} \right) = \log \left( \sinh \theta + \cosh \theta \right).$

    2. Express $\sinh \theta + \cosh \theta$ in exponential form By definition:

    $\sinh \theta = \frac{e^\theta - e^{-\theta}}{2}, \quad \cosh \theta = \frac{e^\theta + e^{-\theta}}{2}.$

    Adding them:

    $\sinh \theta + \cosh \theta = \frac{e^\theta - e^{-\theta} + e^\theta + e^{-\theta}}{2} = \frac{2e^\theta}{2} = e^\theta.$

    3. Take the logarithm Since we are using the natural logarithm (common in such contexts), $\log(e^\theta) = \theta.$ …

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