Number of solutions of the equation tan x + sec x = 2cos x lying in the interval [0, 2π] is (A) 0 (B) 1 (C) 2

NCERT Exemplar · Q53

\tan x + \sec x = 2\cos x

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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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The key is to rewrite $\tan x + \sec x$ as $\frac{1+\sin x}{\cos x}$ , then multiply through and use $\sin^2 x + \cos^2 x = 1$ to get a quadratic in $\sin x$ . After checking for extraneous roots (where $\cos x = 0$ ), only two solutions lie in $[0, 2\pi]$ , so the answer is (C) 2.

Concept & Intuition

When an equation mixes $\tan x$ , $\sec x$ , and $\cos x$ , the natural move is to express everything in terms of $\sin x$ and $\cos x$ . That gives a rational equation — and rational equations can introduce extraneous solutions when we multiply by something that could be zero. Here, multiplying by $\cos x$ is safe only if $\cos x \neq 0$ ; any candidate where $\cos x = 0$ must be discarded. After that, the equation becomes a quadratic in $\sin x$ , which we solve and then check against the interval.

Step-by-step solution

Rewrite in sine and cosine $\tan x = \frac{\sin x}{\cos x}$ , $\sec x = \frac{1}{\cos x}$ . So the equation becomes

\frac{\sin x}{\cos x} + \frac{1}{\cos x} = 2\cos x

Combine the left side:

\frac{\sin x + 1}{\cos x} = 2\cos x

Multiply through — but watch the domain Multiply both sides by $\cos x$ (provided $\cos x \neq 0$ ):

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

Watch out

If $\cos x = 0$ , the original equation has $\tan x$ and $\sec x$ undefined (division by zero). So any solution with $\cos x = 0$ is automatically invalid — we must reject $x = \frac{\pi}{2}, \frac{3\pi}{2}$ .

Use the Pythagorean identity Replace $\cos^2 x$ with $1 - \sin^2 x$ :

\sin x + 1 = 2(1 - \sin^2 x)

Expand:

\sin x + 1 = 2 - 2\sin^2 x

Form a quadratic in $\sin x$ Bring all terms to one side:

2\sin^2 x + \sin x + 1 - 2 = 0

2\sin^2 x + \sin x - 1 = 0

Factor:

(2\sin x - 1)(\sin x + 1) = 0

So $\sin x = \frac{1}{2}$ or $\sin x = -1$ .

Find candidate $x$ in $[0, 2\pi]$
- For $\sin x = \frac{1}{2}$ : $x = \frac{\pi}{6}$ and $x = \frac{5\pi}{6}$ .
- For $\sin x = -1$ : $x = \frac{3\pi}{2}$ .
Check each candidate against the original equation …

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Q.Number of solutions of the equation tan⁡x+sec⁡x=2cos⁡x\tan x + \sec x = 2\cos xtanx+secx=2cosx lying in the interval [0,2π][0, 2\pi][0,2π] is (A) 000 (B) 111 (C) 222 (D) 333

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Q.Number of solutions of the equation $\tan x + \sec x = 2\cos x$ lying in the interval $[0, 2\pi]$ is
(A) $0$
(B) $1$
(C) $2$
(D) $3$

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