Q.If $\sin\theta = \dfrac{4}{5}$ and $\theta$ is not in the first quadrant, find the value of $\cos\theta$.

Showing the 12 most recent of 54 on this concept.

1. CBSE 2026Set ANNUAL1 markMCQ
   Q.The value of $\sin \dfrac{31\pi}{3}$ is

   (a) $\dfrac{\sqrt{3}}{2}$

   (b) $-\dfrac{\sqrt{3}}{2}$

   (c) $\dfrac{1}{2}$

   (d) $\dfrac{1}{\sqrt{2}}$

   ›Reveal solutionSolution

   Subtract multiples of $2\pi$ (period of sine) until the angle lies in $[0, 2\pi)$; $31\pi/3$ reduces to $\pi/3$.

   Since sine has period $2\pi = \dfrac{6\pi}{3}$, subtract multiples of $\dfrac{6\pi}{3}$ from $\dfrac{31\pi}{3}$: …

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2. CBSE 2026Set ANNUAL1 markMCQ
   Q.If $\tan x = \dfrac{3}{4}$, $\pi < x < \dfrac{3\pi}{2}$ then the value of $\sin \dfrac{x}{2}$ is

   (a) $-\dfrac{1}{\sqrt{10}}$

   (b) $\dfrac{1}{\sqrt{10}}$

   (c) $\dfrac{3}{\sqrt{10}}$

   (d) None of these

   ›Reveal solutionSolution

   Find $\cos x$ from $\tan x$ (both sin, cos negative in Q3), then use the half-angle formula, choosing the sign from the quadrant of $x/2$.

   Given $\tan x = \dfrac{3}{4}$ with $\pi < x < \dfrac{3\pi}{2}$ ($x$ in the third quadrant, where both $\sin x$ and $\cos x$ are negative). Using the 3-4-5 triangle: $\sin x = -\dfrac{3}{5}$, $\cos x = -\dfrac{4}{5}$.

   Since $\pi < x < \dfrac{3\pi}{2}$, dividing by 2: $\dfrac{\pi}{2} < \dfrac{x}{2} < \dfrac{3\pi}{4}$ — so $x/2$ lies in the second quadrant, where sine is positive.

   …

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3. CBSE 2026Set ANNUAL1 markMCQ
   Q.Which one of the following is the value of $\tan \frac{19\pi}{3}$?

   (a) $\sqrt{3}$

   (b) $-\sqrt{3}$

   (c) $\frac{1}{\sqrt{3}}$

   (d) $-\frac{1}{\sqrt{3}}$

   ›Reveal solutionSolution

   $\tan\frac{19\pi}{3} = \sqrt{3}$, option (a).

   Since $\tan\theta$ has period $\pi$, we can subtract integer multiples of $\pi$ from the angle without changing its value.

   $\frac{19\pi}{3} - 6\pi = \frac{19\pi}{3} - \frac{18\pi}{3} = \frac{\pi}{3}$ …

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4. CBSE 2026Set ANNUAL1 markMCQ
   Q.If tan x = -5/12 and x lies in 2nd quadrant, then the value of sin x is:

   (a) 5/13

   (b) 12/13

   (c) -5/13

   (d) -12/13

   ›Reveal solutionSolution

   In the 2nd quadrant sine is positive; using the 5-12-13 right triangle from tan x = -5/12 gives sin x = 5/13.

   Given $\tan x = -\dfrac{5}{12}$ with $x$ in the 2nd quadrant.

   In the 2nd quadrant: $\sin x > 0$, $\cos x < 0$, and $\tan x = \dfrac{\sin x}{\cos x} < 0$ — consistent with the given negative value.

   Treat $5$ and $12$ as the magnitudes of the opposite and adjacent sides; the hypotenuse is $\sqrt{5^2+12^2} = \sqrt{25+144} = \sqrt{169} = 13$.

   …

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5. CBSE 2026Set ANNUAL1 markMCQ
   Q.Assertion (A): The function $\sin x$ is negative in the third and fourth quadrant. Reason (R): The function $\sin x$ is decreasing in the interval $0 \le x \le \dfrac{\pi}{2}$.

   (a) Both Assertion (A) and Reason (R) are correct and Reason (R) is the correct explanation of Assertion (A).

   (b) Both Assertion (A) and Reason (R) are correct, but Reason (R) is not the correct explanation of Assertion (A).

   (c) Assertion (A) is correct, but Reason (R) is incorrect.

   (d) Assertion (A) is incorrect, but Reason (R) is correct.

   ›Reveal solutionSolution

   Assertion (A) is true, but Reason (R) is false — $\sin x$ increases, not decreases, on $[0,\pi/2]$ — so (R) cannot even be a valid explanation of (A).

   Checking Assertion (A): In the third quadrant ($\pi < x < \tfrac{3\pi}{2}$) and fourth quadrant ($\tfrac{3\pi}{2} < x < 2\pi$), the value of $\sin x$ is indeed negative (sine is positive only in the first and second quadrants). So (A) is TRUE.

   …

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6. CBSE 2026Set ANNUAL1 markMCQ
   Q.Match the columns — Column A: $\sin(-x)$. Choose its correct equivalent from Column B.

   (a) $\dfrac{1-\tan^2 x}{1+\tan^2 x}$

   (b) $\dfrac{2\tan x}{1-\tan^2 x}$

   (c) $\dfrac{2\tan x}{1+\tan^2 x}$

   (d) $-\sin x$

   (e) $\sin x$

   (f) $-\cos x$

   (g) $\cos x$

   ›Reveal solutionSolution

   Sine is an odd function: $\sin(-x)=-\sin x$ for all $x$.

   This follows directly from the standard trigonometric identity for negative angles, provable from the unit-circle definition: reflecting the angle across the x-axis flips the sign of the y-coordi …

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7. CBSE 2026Set ANNUAL1 markMCQ
   Q.Match the columns — Column A: $\cos\left(\dfrac{\pi}{2} - x\right)$. Choose its correct equivalent from Column B.

   (a) $\dfrac{1-\tan^2 x}{1+\tan^2 x}$

   (b) $\dfrac{2\tan x}{1-\tan^2 x}$

   (c) $\dfrac{2\tan x}{1+\tan^2 x}$

   (d) $-\sin x$

   (e) $\sin x$

   (f) $-\cos x$

   (g) $\cos x$

   ›Reveal solutionSolution

   By the co-function identity, $\cos\left(\dfrac{\pi}{2}-x\right)=\sin x$.

   This is a standard complementary-angle identity: the cosine of $90^\circ$ (i.e. $\tfrac{\pi}{2}$) minus an angle equals the sine of that angle, following from the right-trian …

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8. CBSE 2026Set ANNUAL1 markMCQ
   Q.Match the columns — Column A: $\cos(\pi - x)$. Choose its correct equivalent from Column B.

   (a) $\dfrac{1-\tan^2 x}{1+\tan^2 x}$

   (b) $\dfrac{2\tan x}{1-\tan^2 x}$

   (c) $\dfrac{2\tan x}{1+\tan^2 x}$

   (d) $-\sin x$

   (e) $\sin x$

   (f) $-\cos x$

   (g) $\cos x$

   ›Reveal solutionSolution

   By the supplementary-angle identity, $\cos(\pi-x)=-\cos x$.

   This follows since $\pi-x$ lies in the second quadrant when $x$ is a first-quadrant angle, where cosine is negative, and the reference angle is $x$ itself, …

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9. CBSE 2026Set 1A1 mark
   Q.Find the value of the $\sin\left(-\frac{11\pi}{3}\right)$.

   ›Reveal solutionSolution

   $-\tfrac{11\pi}{3}+4\pi=\tfrac{\pi}{3}$, so the value is $\sin\tfrac{\pi}{3}=\tfrac{\sqrt3}{2}$.

   Using periodicity $\sin\theta=\sin(\theta+2\pi k)$:

   $-\frac{11\pi}{3}+4\pi=-\frac{11\pi}{3}+\frac{12\pi}{3}=\frac{\pi}{3}.$ …

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10. CBSE 2025Set ANNUAL1 markMCQ
    Q.$\sin \dfrac{3\pi}{2} =$

    (a) 0

    (b) -1

    (c) 1

    (d) 1/2

    ›Reveal solutionSolution

    $\sin\dfrac{3\pi}{2} = -1$.

    $\dfrac{3\pi}{2}$ radians $= 270°$. On the unit circle, $270°$ corresponds to the point $(0, -1)$ on the negative $y$-axis, where $\sin\theta$ equal …

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11. CBSE 2025Set ANNUAL1 markMCQ
    Q.$\sin(\pi + x) =$

    (a) $\cos x$

    (b) $-\cos x$

    (c) $\sin x$

    (d) $-\sin x$

    ›Reveal solutionSolution

    $\sin(\pi + x) = -\sin x$.

    …

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12. CBSE 2025Set ANNUAL1 markMCQ
    Q.$\cos \dfrac{5\pi}{2} =$

    (a) 0

    (b) 1

    (c) -1

    (d) 1/2

    ›Reveal solutionSolution

    $\cos\dfrac{5\pi}{2} = 0$.

    Since cosine has period $2\pi$: $\dfrac{5\pi}{2} - 2\pi = \dfrac{5\pi}{2} - \dfrac{4\pi}{2} = \dfrac{\pi}{2}$.

    …

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