# NCERT Class 11-Math's: Chapter 3 Exemplar Trigonometric Functions Part 13

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## Long Answer Type

**Question 20:**

If then prove that

**Answer:**

Given,

prove:

As, LHS

Using:

It involves terms so we need to calculate it first.

Similarly,

Using:

Thus,

**Question 21:**

If , then prove that

**Answer:**

Given,

To prove: .

Applying componendo – dividend, we get

From transformation formula, we know that –

**Question 22:**

Find the value of the expression

**Answer:**

Let,

We know that-