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.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Double-Angle Formulas

**Physics Forums | Science Articles, Homework Help, Discussion**