?? I don't understand this answer 1. The problem statement, all variables and given/known data The pdf attached is the answer to a question from a Calculus quiz. (the right hand side is a bit cut off, but the formula is repeated further down) Edit: the "f(x) = ..., and a=0" part is the solution, not part of the the question. 2. Relevant equations 3. The attempt at a solution ok. I have no idea what she's doing here. when I do the derivative of the f(x) she gives as the answer, I get 1/2 (1+3cosx)^1/2 is 1/4 ((1+3cosx)^-1/2)(-3sinx) = -3sinx / 4 sqrt(1+3cosx) which is not even close to the original f'(x) she posted... can anyone show me how she got that answer? thanks.