# Introduction to Trigonometry – Class X – Part 7 | Exercise 8.4 Q1-3 Solved

### Watch the seventh video session on “Introduction To Trigonometry” for Class Xth – concepts and solved questions

Introduction To Trigonometry – Class X – Part 7

1. Trigonometric identities that are in syllabus (class 10, NCERT):

sin²θ + cos²θ = 1

1 + cot²θ = cosec²θ

1 + tan²θ = sec²θ

2. How to remember trigonometric identities: As explained in the video, just remember the first one i.e sin²θ + cos²θ = 1

a) The second one (1 + cot²θ = cosec²θ) can be derived from the first by dividing both sides by sin²θ.

b) The second one (1 + tan²θ = sec²θ) can be derived from the first by dividing both sides by cos²θ.

3. Usually the concepts of “trigonometric angles for complementary angles” and “trigonometric identities” are tested together in exam questions.

NCERT Exercise 8.4 Q1-3 Solved

1. Express the trigonometric ratios sin A, sec A and tan A in terms of cot A.

2. Write all the other trigonometric ratios of ∠ A in terms of sec A.

3. Evaluate :

(i) (sin²63° + sin²27°) / (cos²17° + cos²73°)

(ii) sin 25°cos 65° + cos 25°sin 65°

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