Question

cho \(cosa=\frac{3}{4}\). tính \(cos\frac{3a}{2}.cos\frac{a}{2}\)

Answer 1

\(cos\frac{3a}{2}.cos\frac{a}{2}=\frac{1}{2}\left(cos2a+cosa\right)=\frac{1}{2}\left(2cos^2a-1+cosa\right)\)

\(=\frac{1}{2}\left(2.\frac{9}{16}-1+\frac{3}{4}\right)=...\)