Prove the side of triangle is is produced then the exterior angle formed is equal to the sum of two interior opposite angle

Answer 1

recall that alternate interior angles between parallel lines are congruent.

Draw a line through one vertex parallel to the opposite base.

the rest is easy.

Answer 2

let the angles in the interior of the triangle be
x, y, and z
Extend the base at the z angle
let the exterior angle be k
we know x+y+z = 180°
we know that the z + k = 180 , (angles forming a straight line)

so x+y+z = z + k
x + y = k

state your conclusion