This is actually part of a calculus problem, but the part I was struggling with was pure trig, so here you are:
The back of the book simplifies this to:
How?
Do you happen to know what the general term for this kind of equation is, so I can look it up in my trig book? The problem is when I went to calculus, I skipped trig, so some of these basic "you should know" things are completely new to me.
Also confused about how you did the last line:
there is no general term you should memories formulas
memories the formula in this link not all but the important ones like
something like that
Math Forum: Ask Dr. Math FAQ: Trigonometry Formulas
the thing that you are confused from it , I just multiply 2(2x) = 4x what hard in this .