# Precal/trig

sina=3/5, pi/2<a<pi evaluate cos2a

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1. cos2a = 1 - 2sin^2(a)
= 1 - 2*9/25
...

in this case, it doesn't really matter which quadrant a is in, as sin^2 is always positive.

extra credit: which quadrant is 2a in?

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posted by Steve

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