(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 intersecting one's coz in 3d we have skew lines for which angle between lines has no meaning as it can't be found coz they don't intersect
