



 
Hi Shanaia, Use the fact that $\sin^2 A + \cos^2 A = 1$ to find $\cos A.$ How do you know which sign to use? Now use the fact that $\cos 2B = \cos^2 B  \sin^2 B = 1  2 \sin^2 B$ with $ B = \large \frac{A}{2}.$ Penny  


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