If $\tan\alpha = \dfrac{1}{7}$, $\tan\beta = \dfrac{1}{3}$, then $\cos 2\alpha$ is equal to (A) $\sin 2\beta$ (B) $\sin 4\beta$ (C) $\sin 3\beta$ (D) $\cos 2\beta$

> [!TLDR] > We calculate $\cos 2\alpha$ using the given $\tan\alpha$ and find it to be $\frac{24}{25}$. Then, we evaluate the options using $\tan\beta$ and discover that $\sin 4\beta$ also equals $\frac{24}{25}$. The correct option is **(B)**. The core idea here is to use the double angle trigonometric identities that express $\cos 2\theta$ and $\sin 2\theta$ in terms of $\tan \theta$. Since we are given the values of $\tan\alpha$ and $\tan\beta$, these specific formulas are the most direct and efficient way to solve the problem, avoiding the need to calculate $\sin\alpha$, $\cos\alpha$, $\sin\beta$, or $\cos\beta$ individually. We will first calculate the value of $\cos 2\alpha$ using the given $\tan\alpha$. Then, we will evaluate each option using the given $\tan\beta$ until we find a match. 1. **Calculate $\cos 2\alpha$ using $\tan\alpha$** We are given $\tan\alpha = \frac{1}{7}$. The double angle formula for cosine in terms of tangent is: > [!FORMULA] > $\cos 2\theta = \frac{1 - \tan^2\theta}{1 + \tan^2\theta}$ Substitute $\tan\alpha = \frac{1}{7}$ into this formula: $$ \cos 2\alpha = \frac{1 - \left(\frac{1}{7}\right)^2}{1 + \left(\frac{1}{7}\right)^2} $$ $$ \cos 2\alpha = \frac{1 - \frac{1}{49}}{1 + \frac{1}{49}} $$ To simplify, find a common denominator for the numerator and denominator: $$ \cos 2\alpha = \frac{\frac{49}{49} - \frac{1}{49}}{\frac{49}{49} + \frac{1}{49}} $$ $$ \cos 2\alpha = \frac{\frac{48}{49}}{\frac{50}{49}} $$ Now, we can cancel out the common denominator $49$: $$ \cos 2\alpha = \frac{48}{50} = \frac{24}{25} $$ So, we need to find an option that evaluates to $\frac{24}{25}$. 2. **Evaluate the options using $\tan\beta$** We are given $\tan\beta = \frac{1}{3}$. We will check the options one by one. * **Option (A): $\sin 2\beta$** The double angle formula for sine in terms of tangent is: > [!FORMULA] > $\sin 2\theta = \frac{2\tan\theta}{1 + \tan^2\the

If tanα = (1)/(7), tanβ = (1)/(3), then cos 2α is equal to (A) sin 2β (B) sin 4β (C) sin 3β (D) cos 2β

NCERT Exemplar · Q57

\tan\alpha = \dfrac{1}{7}

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Concept understanding — Trigonometric Identity Proof

Trigonometric Identity Proof: From Intuition to Precision

Imagine you're standing at the corner of a right triangle. The two shorter sides — one horizontal, one vertical — and the sloping hypotenuse are all connected. If you change the angle at your corner, the lengths of the sides change, but the relationship between them stays fixed. That fixed relationship is what a trigonometric identity captures.

The Core Idea

A trigonometric identity is an equation involving trigonometric functions (like $\sin \theta$ , $\cos \theta$ , $\tan \theta$ ) that is true for every angle $\theta$ where both sides are defined. It's not a conditional equation (like $\sin \theta = 0.5$ , which is true only for specific angles). It's an eternal truth about how these functions relate.

The most famous one is:

\sin^2 \theta + \cos^2 \theta = 1

This holds for any angle $\theta$ — acute, obtuse, negative, whatever. Why? Because on the unit circle, $\sin \theta$ is the $y$ -coordinate and $\cos \theta$ is the $x$ -coordinate of a point on a circle of radius 1. The Pythagorean theorem says $x^2 + y^2 = 1$ , so $\sin^2 \theta + \cos^2 \theta = 1$ is just the Pythagorean theorem in disguise.

Proving an Identity: The Method

When you're asked to prove a trigonometric identity, you're not solving for an angle. You're showing that the left-hand side (LHS) and right-hand side (RHS) are the same expression, just written differently.

The golden rule: Start with one side and transform it into the other, using known identities and algebraic manipulation. Never move terms across the equals sign as if solving an equation — that assumes the identity is already true, which is what you're trying to prove.

A Simple Example

Prove: $\tan \theta \cdot \cos \theta = \sin \theta$

Step 1: Pick a side to start with. Usually, the more complicated side is easier to simplify. Here, the LHS looks more complex.

Step 2: Replace $\tan \theta$ with $\frac{\sin \theta}{\cos \theta}$ (a known identity).

\tan \theta \cdot \cos \theta = \frac{\sin \theta}{\cos \theta} \cdot \cos \theta

Step 3: Cancel $\cos \theta$ (provided $\cos \theta \neq 0$ — but the identity holds for all angles where both sides are defined, and at $\cos \theta = 0$ , $\tan \theta$ is undefined anyway).

= \sin \theta

That's it. The LHS simplifies exactly to the RHS.

The Toolbox of Known Identities

To prove any identity, you need to know the basic building blocks:

Identity	Formula
Pythagorean	$\sin^2 \theta + \cos^2 \theta = 1$
Quotient	$\tan \theta = \frac{\sin \theta}{\cos \theta}$ , $\cot \theta = \frac{\cos \theta}{\sin \theta}$
Reciprocal	$\csc \theta = \frac{1}{\sin \theta}$ , $\sec \theta = \frac{1}{\cos \theta}$ , $\cot \theta = \frac{1}{\tan \theta}$
Even-Odd	$\sin(-\theta) = -\sin \theta$ , $\cos(-\theta) = \cos \theta$

Watch out

A common mistake is to treat $\sin^2 \theta$ as $(\sin \theta)^2$ — which it is — but then incorrectly think $\sin^2 \theta + \cos^2 \theta = 1$ means $\sin \theta + \cos \theta = 1$ . It does not. The square applies to the whole sine value, not to the angle.

A Slightly Harder Proof

Prove: $\frac{1 - \cos^2 \theta}{\cos \theta} = \sin \theta \tan \theta$

Start with LHS: $\frac{1 - \cos^2 \theta}{\cos \theta}$

From the Pythagorean identity, $1 - \cos^2 \theta = \sin^2 \theta$ . So:

\frac{\sin^2 \theta}{\cos \theta} = \sin \theta \cdot \frac{\sin \theta}{\cos \theta} = \sin \theta \tan \theta

That's the RHS. Done. …

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We calculate $\cos 2\alpha$ using the given $\tan\alpha$ and find it to be $\frac{24}{25}$ . Then, we evaluate the options using $\tan\beta$ and discover that $\sin 4\beta$ also equals $\frac{24}{25}$ . The correct option is (B).

The core idea here is to use the double angle trigonometric identities that express $\cos 2\theta$ and $\sin 2\theta$ in terms of $\tan \theta$ . Since we are given the values of $\tan\alpha$ and $\tan\beta$ , these specific formulas are the most direct and efficient way to solve the problem, avoiding the need to calculate $\sin\alpha$ , $\cos\alpha$ , $\sin\beta$ , or $\cos\beta$ individually.

We will first calculate the value of $\cos 2\alpha$ using the given $\tan\alpha$ . Then, we will evaluate each option using the given $\tan\beta$ until we find a match.

Calculate $\cos 2\alpha$ using $\tan\alpha$

We are given $\tan\alpha = \frac{1}{7}$ .

The double angle formula for cosine in terms of tangent is:
$\cos 2\theta = \frac{1 - \tan^2\theta}{1 + \tan^2\theta}$
Substitute $\tan\alpha = \frac{1}{7}$ into this formula:

\cos 2\alpha = \frac{1 - \left(\frac{1}{7}\right)^2}{1 + \left(\frac{1}{7}\right)^2}

\cos 2\alpha = \frac{1 - \frac{1}{49}}{1 + \frac{1}{49}}

To simplify, find a common denominator for the numerator and denominator:

\cos 2\alpha = \frac{\frac{49}{49} - \frac{1}{49}}{\frac{49}{49} + \frac{1}{49}}

\cos 2\alpha = \frac{\frac{48}{49}}{\frac{50}{49}}

Now, we can cancel out the common denominator $49$:

\cos 2\alpha = \frac{48}{50} = \frac{24}{25}

So, we need to find an option that evaluates to $\frac{24}{25}$.

2. Evaluate the options using $\tan\beta$

We are given $\tan\beta = \frac{1}{3}$ . We will check the options one by one.

*   **Option (A): $\sin 2\beta$**
    The double angle formula for sine in terms of tangent is:
    > [!FORMULA]
    > $\sin 2\theta = \frac{2\tan\theta}{1 + \tan^2\theta}$

    Substitute $\tan\beta = \frac{1}{3}$ into this formula:

\sin 2\beta = \frac{2\left(\frac{1}{3}\right)}{1 + \left(\frac{1}{3}\right)^2}

\sin 2\beta = \frac{\frac{2}{3}}{1 + \frac{1}{9}}

\sin 2\beta = \frac{\frac{2}{3}}{\frac{9}{9} + \frac{1}{9}}

\sin 2\beta = \frac{\frac{2}{3}}{\frac{10}{9}}

    To divide fractions, multiply by the reciprocal of the denominator:

\sin 2\beta = \frac{2}{3} \times \frac{9}{10}

\sin 2\beta = \frac{18}{30} = \frac{3}{5}

    Since $\frac{3}{5} \neq \frac{24}{25}$, option (A) is not correct.

*   **Option (D): $\cos 2\beta$**
    Let's check this option next, as it uses the same type of formula as $\cos 2\alpha$.
    Using the formula $\cos 2\theta = \frac{1 - \tan^2\theta}{1 + \tan^2\theta}$ with $\theta = \beta$: …

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Q.If tan⁡α=17\tan\alpha = \dfrac{1}{7}tanα=71​, tan⁡β=13\tan\beta = \dfrac{1}{3}tanβ=31​, then cos⁡2α\cos 2\alphacos2α is equal to (A) sin⁡2β\sin 2\betasin2β (B) sin⁡4β\sin 4\betasin4β (C) sin⁡3β\sin 3\betasin3β (D) cos⁡2β\cos 2\betacos2β

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Q.If $\tan\alpha = \dfrac{1}{7}$ , $\tan\beta = \dfrac{1}{3}$ , then $\cos 2\alpha$ is equal to
(A) $\sin 2\beta$
(B) $\sin 4\beta$
(C) $\sin 3\beta$
(D) $\cos 2\beta$

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