Đáp án:
$\cos\dfrac{3a}{2}.\cos\dfrac{a}{2}=\dfrac{7}{16}$
Giải thích các bước giải:
$\cos\dfrac{3a}{2}.\cos\dfrac{a}{2}$
$=\dfrac{1}{2}.\left[\cos\left(\dfrac{3a}{2}+\dfrac{a}{2}\right)+\cos\left(\dfrac{3a}{2}-\dfrac{a}{2}\right)\right]$
$=\dfrac{1}{2}.\left(\cos2a+\cos a\right)$
$=\dfrac{1}{2}.(2\cos^2a-1+\cos a)$
$=\dfrac{1}{2}.\left[2.\left(\dfrac{3}{4}\right)^2+\dfrac{3}{4}-1\right]$
$=\dfrac{7}{16}$.
