Question

Which of these is equal to sin 21°?

A) cos 21°
B) cos 69°
C) sin 69°
D) tan 69°

Answer 1

Answer: Choice B) cos(69)

Work Shown:
cos(90-x) = sin(x)
cos(90-21) = sin(21)
cos(69) = sin(21)

Answer 2

B. Cos 69°

Explanation:

Given : sin 21°

Solution : To show how Cos 69 ° = Sin 21 °

we will use sine and cosine trigonometric identity

i.e. Cos (90 °-x) = Sin x ---(A)

We are given Sin 21 °

So, putting x =21 ° in (A)

We are getting : Cos(90 °-21 °) = Sin 21 °

⇒Cos 69 ° = Sin 21 °

Hence it is shown that Cos 69 ° = Sin 21 °