If $a\cos\theta + b\sin\theta = m$ and $a\sin\theta - b\cos\theta = n$, then show that $a^2 + b^2 = m^2 + n^2$.

If acosθ + bsinθ = m and asinθ - bcosθ = n, then s – Class 11 Mathematics

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a\cos\theta + b\sin\theta = m

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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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Square both equations and add them; the cross-terms cancel by the Pythagorean identity $\cos^2\theta + \sin^2\theta = 1$ , leaving $a^2 + b^2 = m^2 + n^2$ .

The heart of this problem is recognizing that when you square linear combinations of sine and cosine, the mixed terms vanish upon addition because of the fundamental identity $\cos^2\theta + \sin^2\theta = 1$ . This is a classic technique in trigonometry: whenever you see expressions like $a\cos\theta + b\sin\theta$ paired with a similar expression, squaring and adding often eliminates the trigonometric functions entirely.

Why does this work? The two given equations are orthogonal in a sense—one involves $\cos\theta$ with $a$ and $\sin\theta$ with $b$ , while the other swaps the roles with opposite signs. This structure is designed so that when squared and summed, the coefficients of $\cos^2\theta$ and $\sin^2\theta$ align perfectly to factor out $a^2$ and $b^2$ .

1. Square the first equation

Starting with $a\cos\theta + b\sin\theta = m$ , square both sides:

(a\cos\theta + b\sin\theta)^2 = m^2

Expand the left side:

a^2\cos^2\theta + 2ab\cos\theta\sin\theta + b^2\sin^2\theta = m^2

2. Square the second equation

Now take $a\sin\theta - b\cos\theta = n$ and square both sides:

(a\sin\theta - b\cos\theta)^2 = n^2

Expand:

a^2\sin^2\theta - 2ab\sin\theta\cos\theta + b^2\cos^2\theta = n^2

3. Add the two squared equations

Add the results from steps 1 and 2: …

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Q.If acos⁡θ+bsin⁡θ=ma\cos\theta + b\sin\theta = macosθ+bsinθ=m and asin⁡θ−bcos⁡θ=na\sin\theta - b\cos\theta = nasinθ−bcosθ=n, then show that a2+b2=m2+n2a^2 + b^2 = m^2 + n^2a2+b2=m2+n2.

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Q.If $a\cos\theta + b\sin\theta = m$ and $a\sin\theta - b\cos\theta = n$ , then show that $a^2 + b^2 = m^2 + n^2$ .

Unlock everything free for 14 days