Triple angle formula

Sara
student 12th grade

Can one derive a triple angle formula for sine and cosine? If so, how?

Hi Sara,
There are many easy ways, but since you probably know the formulas for the sine and cosine of 2x and of a+b, just use those formulas on 3x = 2x + x:

sin 3x = 3sin x - 4(sin³x)
cos 3x = 4(cos x)³ - 3cos x. Cheers,
Chris Go to Math Central