Answer
$=\cos {10}\alpha $
Work Step by Step
$\cos \dfrac {A}{2}=\pm \sqrt {\dfrac {1+\cos A}{2}}\Rightarrow \pm \sqrt {\dfrac {1+\cos {20}\alpha }{2}}=\cos \dfrac {20\alpha }{2}=\cos {10}\alpha $
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.