Write Cos 20° in terms of sin.?

I don't get it, i tried watching videos but still gets me nowhere. pls help

1 Answer
Apr 3, 2018

Sine and cosine are cofunctions. Therefore, the relationship sinx = cos(90 - x) and cosx = sin(90 - x) applies.
cos(20) = sin(90 - 20) = sin(70)

Explanation:

Cofunctions are pairs of trigonometric functions with the simple relationship, where, for example, sin(alpha) = cos(beta) where alpha + beta = 90. This also holds true for csc and sec, and cot and tan.

This means that any cos(alpha) is always equal to sin(90 - alpha), or vice versa.