Complete the proof of the exterior angle theorem given angle 4 is an exterior angle of triangle ABC prove angle 1 + angle 2 = angle 4

In the triangle, 1,2,3 are interior angles and 4 is the exterior angle, adjascent to 3

angle 1 + angle 2 + angle 3 = * 180

angle 3+angle 4=*180

so, we can say,

angle 1+angle 2+angle 3 = angle 3+angle 4

i, e; angle 1+angle 2 = angle 4 proved.