Tính: A = cos 20°. cos 40° . cos 60°. cos 80°.

Ta có A = cos 20°. cos 40° . cos 60°. cos 80°

Suy ra A.sin 20° = sin 20° . cos 20°. cos 40° . cos 60°. cos 80°

= \(\frac{1}{2}\)sin 40° . cos 40° . cos 60°. cos 80°

=\(\frac{1}{4}\) sin80° . cos 60°. cos 80°

= \(\frac{1}{8}\) sin 160° . cos60°

= \(\frac{1}{8}\)sin 20° .cos60°

Do đó A = \(\frac{1}{8}\)cos 60° = \(\frac{1}{{16}}\).