#### Answer

$\cos 30=\dfrac {\sqrt {3}}{2}$

#### Work Step by Step

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

Published by
Pearson

ISBN 10:
013421742X

ISBN 13:
978-0-13421-742-0

$\cos 30=\dfrac {\sqrt {3}}{2}$

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