Q.Prove that $(\cos x + \cos y)^2 + (\sin x - \sin y)^2 = 4\cos^2\frac{x+y}{2}$.

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:

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).

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).

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:

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:

That's the RHS. Done.

What Makes a Proof Valid?

1. Every step must be reversible or an equivalence. You're not solving; you're rewriting.
2. State any restrictions. If you divide by $\cos \theta$, note that $\cos \theta \neq 0$ for that step — but the identity may still hold in the limit.
3. Work on one side only. The cleanest proofs transform LHS into RHS (or vice versa) without touching both sides simultaneously.

The Big Picture

Trigonometric identities are the grammar of trigonometry. They let you simplify complex expressions, solve equations, and later integrate trigonometric functions in calculus. Every proof is just a puzzle: "Can I connect these two expressions using the relationships I already know?"

Start with the simplest identity — $\sin^2 \theta + \cos^2 \theta = 1$ — and build from there. With practice, you'll see the patterns: factor, substitute, cancel, rewrite. That's all there is to it.

Proving trigonometric identities using the Pythagorean, quotient, and reciprocal relations is a staple exercise in the NCERT Class 11 Mathematics chapter on Trigonometric Functions, and "how to prove trigonometric identities step by step" is a commonly searched topic for CBSE board and JEE Main preparation. Because these identities are reused throughout calculus and coordinate geometry, they are consistently featured in "trigonometric identities important questions" for competitive-exam practice.