Đáp án:
B
Giải thích các bước giải:
$\cos2a=2\cos^2a-1\\
\Rightarrow 2\cos^2a=\cos2a+1\\
=\dfrac{1}{4}+1=\dfrac{5}{4}\\
\Rightarrow \cos^2a=\dfrac{5}{8}\\
+)\sin^2a+\cos^2a=1\\
\Rightarrow \sin^2a=1-\cos^2a=1-\dfrac{5}{8} =\dfrac{3}{8}\\
\Rightarrow \sin a=\dfrac{\sqrt{6}}{4}\\
+) \sin2a\cos a=2\sin a\cos a\cos a\\
=2\sin a\cos^2a\\
=2.\dfrac{\sqrt{6}}{4}.\dfrac{5}{8}\\
=\dfrac{5\sqrt{6}}{16}$
