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.(adsbygoogle = window.adsbygoogle || []).push({});

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.

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# Double-Angle Formulas

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