Question

prove that the bisector of the angle of a linear pair are at right angles.

Answer 1

let 1st angle be X
2nd angle be X
because both angle at equal

(by linear pair) X+X=180
2X=180
X=90
hence each will be 90

Answer 2

a linear pair ads up to 180
A angular bisector divides angle into equal halves
therefore EACH angle 90