The angle between two diagonals of a cube is

$\cos \theta=\frac{\sqrt{3}}{2}$ $\cos \theta=\frac{1}{\sqrt{2}}$ $\cos \theta=1 / 3$ none of these

$\cos \theta=1 / 3$

$\cos \theta=\frac{-a^2+a^2+a^2}{\sqrt{a^2+a^2+a^2} \sqrt{a^2+a^2+a^2}}$ of side is a $=\frac{1}{3}$