Có $\sin 10^{o}=\cos 80^{o};\sin 50^{o}=\cos 40^{o};\sin 70^{o}=\cos 20^{o}$$\Rightarrow VT=\cos 20^{o}.\cos 40^{o}.\cos 80^{o}\Rightarrow \sin 20^{o}.VT=\frac{1}{2}.\sin 40^{o}.\cos 40^{o}\cos 80^{o}$
$\Rightarrow \sin 20^{o}.VT=\frac{1}{4}.\sin 80^{o}\cos 80^{o}=\frac{1}{8}.\sin 160^{o}$
Mà $\sin 20^{o}=\sin 160^{o}\Rightarrow VT=\frac{1}{8}$