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°