Answer
$\cos 30=\dfrac {\sqrt {3}}{2}$
Work Step by Step
$\cos ^{2}\alpha =\cos ^{2}\alpha -\sin ^{2}\alpha =1-2\sin ^{2}\alpha \Rightarrow 1-2\sin ^{2}15=\cos \left( 2\times 15\right) =\cos 30=\dfrac {\sqrt {3}}{2}$
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