Answer
$\sin 20$
Work Step by Step
$\sqrt {\dfrac {1-\cos 40}{2}}=\sqrt {\dfrac {1-\cos \left( 2\times 20\right) }{2}}=\sqrt {\dfrac {1-\left( \cos ^{2}20-\sin ^{2}20\right) }{2}}=\sqrt {\dfrac {2\sin ^{2}20}{2}}=\sin 20$
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