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NCERT solution class 10 chapter 8 Introduction to Trigonometry exercise 8.2 mathematics

EXERCISE 8.2


Page No 187:

Question 1:

Evaluate the following

(i) sin60° cos30° + sin30° cos 60°

(ii) 2tan245° + cos230° − sin260°

(iii) 

(iv) 

(v)

Answer:

(i) sin60° cos30° + sin30° cos 60°

(ii) 2tan245° + cos230° − sin260°

(iii) 

(iv) 

(v) 


Question 2:

Choose the correct option and justify your choice.

(i) 

(A). sin60°

(B). cos60°

(C). tan60°

(D). sin30°

(ii) 

(A). tan90°

(B). 1

(C). sin45°

(D). 0

(iii) sin2A = 2sinA is true when A =

(A). 0°

(B). 30°

(C). 45°

(D). 60°

(iv) 

(A). cos60°

(B). sin60°

(C). tan60°

(D). sin30°

Answer:

(i) 

Out of the given alternatives, only 

Hence, (A) is correct.

(ii)

Hence, (D) is correct.

(iii)Out of the given alternatives, only A = 0° is correct.

As sin 2A = sin 0° = 0

2 sinA = 2sin 0° = 2(0) = 0

Hence, (A) is correct.

(iv) 

Out of the given alternatives, only tan 60° 

Hence, (C) is correct.


Question 3:

If  and;

0° < A + B ≤ 90°, A > B find A and B.

Answer:

⇒ 

⇒ A + B = 60 … (1)

⇒ tan (A − B) = tan30

⇒ A − B = 30 … (2)

On adding both equations, we obtain

2A = 90

⇒ A = 45

From equation (1), we obtain

45 + B = 60

B = 15

Therefore, ∠A = 45° and ∠B = 15°


Question 4:

State whether the following are true or false. Justify your answer.

(i) sin (A + B) = sin A + sin B

(ii) The value of sinθ increases as θ increases

(iii) The value of cos θ increases as θ increases

(iv) sinθ = cos θ for all values of θ

(v) cot A is not defined for A = 0°

Answer:

(i) sin (A + B) = sin A + sin B

Let A = 30° and B = 60°

sin (A + B) = sin (30° + 60°)

= sin 90°

= 1

sin A + sin B = sin 30° + sin 60°

Clearly, sin (A + B) ≠ sin A + sin B

Hence, the given statement is false.

(ii) The value of sin θ increases as θ increases in the interval of 0° < θ < 90° as

sin 0° = 0

sin 90° = 1

Hence, the given statement is true.

(iii) cos 0° = 1

cos90° = 0

It can be observed that the value of cos θ does not increase in the interval of 0° < θ < 90°.

Hence, the given statement is false.

(iv) sin θ = cos θ for all values of θ.

This is true when θ = 45°

As 

It is not true for all other values of θ.

As  and ,

Hence, the given statement is false.

(v) cot A is not defined for A = 0°

As ,

 = undefined

Hence, the given statement is true.


 

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