If you want the proof of any particular one let me know thru comments and we can find it.

Notice it resembles one of the sum-difference formulas.

sin(a /- b) = sin(a)cos(b) /- cos(a)sin(b)

which means

sin(a b) = sin(a)cos(b) cos(a)sin(b)

sin(a b) = sin(a)cos(b) cos(a)sin(b)

So

a = 9x

b = x

The one you want is for the sum of two angles.

http://www.sosmath.com/trig/Trig5/trig5/trig5.html

Unfortunatelytrig identities just have to be memorized. It takes too long to prove each one again during a test.

If you want the proof of any particular one let me know thru comments and we can find it.

The one you want is for the sum of two angles.

This post is last updated on hrtanswers.com at Date : 1st of September – 2022