**Toolbox:**

- If $ l, m , n$ are the d.c.s of a vector, then $ l^2+m^2+n^2=1$

$\cos^245^{\circ}+\cos^260^{\circ}+\cos^2120^{\circ}= \large\frac{1}{2}+\large\frac{1}{4}+\large\frac{1}{4}=1$

$ l^2+m^2+n^2=1.$ A vector can have $ 45^{\circ}, 60^{\circ}, 120^{\circ}$ as direction angles.