# NCERT solution class 10 chapter 8 Introduction to Trigonometry exercise 8.4 mathematics

## EXERCISE 8.4

#### Question 1:

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

We know that,

will always be positive as we are adding two positive quantities.

Therefore,

We know that,

However,

Therefore,

Also,

#### Question 2:

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

We know that,

Also, sin2 A + cos2 A = 1

sin2 A = 1 − cos2 A

tan2A + 1 = sec2A

tan2A = sec2A − 1

#### Question 3:

Evaluate

(i)

(ii) sin25° cos65° + cos25° sin65°

(i)

(As sin2A + cos2A = 1)

= 1

(ii) sin25° cos65° + cos25° sin65°

= sin225° + cos225°

= 1 (As sin2A + cos2A = 1)

#### Question 4:

Choose the correct option. Justify your choice.

(i) 9 sec2 A − 9 tan2 A =

(A) 1

(B) 9

(C) 8

(D) 0

(ii) (1 + tan θ + sec θ) (1 + cot θ − cosec θ)

(A) 0

(B) 1

(C) 2

(D) −1

(iii) (secA + tanA) (1 − sinA) =

(A) secA

(B) sinA

(C) cosecA

(D) cosA

(iv)

(A) secA

(B) −1

(C) cotA

(D) tanA

(i) 9 sec2A − 9 tan2A

= 9 (sec2A − tan2A)

= 9 (1) [As sec2 A − tan2 A = 1]

= 9

Hence, alternative (B) is correct.

(ii)

(1 + tan θ + sec θ) (1 + cot θ − cosec θ)

Hence, alternative (C) is correct.

(iii) (secA + tanA) (1 − sinA)

= cosA

Hence, alternative (D) is correct.

(iv)

Hence, alternative (D) is correct.

#### Question 5:

Prove the following identities, where the angles involved are acute angles for which the expressions are defined.

(i)

(ii)

(iii)

= secθ cosec θ +

= R.H.S.

(iv)

= R.H.S

(v)

Using the identity cosec2 = 1 + cot2,

L.H.S =

= cosec A + cot A

= R.H.S

(vi)

(vii)

(viii)

(ix)

Hence, L.H.S = R.H.S

(x)

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