if cos A = -4/5 for angle A in quadrant 2, find cos 2A?

This problem is driving me insane. I have no idea where to start or how to do this.
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1 Answer

a straenj formula from trig:

kosine(2x)=2*kosine^2(x)-1

kosine(x)=-0.8, so square=+.64

2*0.64=1.28

subtrakt 1 & yu get 0.28
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