Find the angle between the lines whose direction ratios are a, b, c and $b - c,c - a,a - b.$

.: The angle between the given lines is given by

$\cos \theta = \cfrac{{a(b - c) + b(c - a) + c(a - b)}}{{\sqrt {{a^2} + {b^2} + {c^2}} \sqrt {{{(b - c)}^2} + {{(c - a)}^2} + {{(a - b)}^2}} }} = 0$

Hence,$\theta = 90^\circ$ .