Question

In theorem 19 prove angle A=C by using theorem 9theorem 9 says if two angles have their sides parallel right side to right side and left side to left side then angles are equal

Answer 1

Corresponding Angles

When two lines are crossed by another line (which is called the Transversal), the angles in matching corners are called corresponding angles.

as you can see from the image, all angles a are equal. same goes for b.

in this case, CB and AD are parallel lines, then angles A and C must be equal.