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OpenStudy (anonymous):

cos2a =???

OpenStudy (unklerhaukus):

\[\cos(2a)=2\cos^2(a)-1=1-2\sin^2(a)=\cos^2(a)-\sin(a)\]

OpenStudy (anonymous):

\[\cos ^{2}a-\sin ^{2}a=1-2\sin ^{2}a=2\cos ^{2}a-1\]

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