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.