Đáp án:
$A=\frac{3}{16}$
Giải thích các bước giải:
$A=cos 10^{\circ}.cos30^{\circ} . cos50^{\circ}.cos70^{\circ}\\
= cos 10^{\circ}.cos30^{\circ} .\frac{1}{2} (cos120^{\circ}+cos20^{\circ})\\
= cos 10^{\circ}.\frac{\sqrt{3}}{2}.\frac{1}{2} (\frac{-1}{2}+cos20^{\circ})\\
= \frac{\sqrt{3}}{4}\left (\frac{-cos10^{\circ}}{2}+\frac{cos30^{\circ}+10^{\circ}}{2} \right )\\
=\frac{\sqrt{3}}{4}.\frac{cos30^{\circ}}{2}=\frac{3}{16}$
