Why does sin(90 + angle ) =cos angle because on using above property it should br - cos angle?

1 Answer
Jun 27, 2018

See below.

Explanation:

The answer comes from the trig identity development:

sin (90 + x) = sin 90 * cos x + cos 90 * sin x

Note that

sin 90 = 1" " and " " cos 90 = 0

Therefore:

sin (90 + x) = cos x

In below example, the complementary arc gives a negative cos value.

sin (x - 90) = sin x * cos 90 - sin 90 * cos x

Note that:

cos 90 = 0 " " and " " sin 90 = 1

Therefore:

sin (x - 90) = - cos x