Question

Calculate without resorting to table.
$\cos {10}^{\circ }\cos {50}^{\circ }\cos {70}^{\circ }$

Answer 1

We know that, $\cos A\cos (60+A)\cos (60-A)=\frac{1}{4}\cos 3A$

put $A=10,$ we get

$\cos 10°\cos 50°\cos 70°=\frac{1}{4}\cos 30°=\frac{\sqrt{3}}{8}$