Originally Posted by

**sinewave85** This is part of a larger example problem that I came across in reviewing. I can't for the life of me remember the identity or principle that would get one from the first form to the second in one step. Maybe it is not one step -- the authors of the textbook eliminated what they assumed one would see automatically? I realize this may be a stupid question, but it is driving me crazy. Anyone?

$\displaystyle \cos{(\pi-2\theta)} = \frac{20-x}{x}$

$\displaystyle \cos{(2\theta)} = \frac{x-20}{x}$