For some reason, I cannot comprehend the concepts behind this. I read the example problems over and over; however, I still cannot understand the process when I go to study or do work on it. Just to refresh your minds, the double-angle formulas: sin2x = 2(sinx)(cosx) cos2x = cos^2x - sin^2x = 1 - 2sin^2x = 2cos^2x-1 tan2x = 2tanx/1-tan^2x The book example: If cosx = -2/3 and x is in quadrant II; find sin2x and cos2x. If someone could explain the processes when using these formulas to solve problems, I'd greatly appreciate it. The book just isn't helping me any.