cos 3x
The answer at the back of the book says
2 sinē (1.5x) - 1
but how do you get that?
Isn't the "Double Angle Formula" for Cosine supposed to follow
cos 2Θ = 2 cosēΘ - 1 ??? Why does the answer say "sin"? And how do they know to use 1.5x? (the book is known to have many typos, if that helps...)