(https://www.teachoo.com/3504/753/Ex-11.2--2---Show-line-(1---1--2)--(3--4---2)-is-perpendicular/category/Ex-11.2/)
But 1st we need to prove that lines are intersecting one's coz in 3d we have possibility of skew lines for which angle between lines has no meaning as it can't be found coz they don't intersect.
