Joshua,
That's good news because cos(3x) ≠ cos^{3}X - cosX sin^{2}X. What is true is that
cos(3x) = cos^{3}X - 3 cosX sin^{2}X
To prove this I wrote cos(3X) as cos(2X + X) and used the identity that
cos(A + B) = cos A cos B - sin A sin B
In simplifying the expression above I need to use
sin(A + B) = sin A cos B + cos A sin B
Penny |