Proof:

L1 // L2 
A1 = A   
B1 = B   

C = 180 - A1 - B1     

A + B + C = A + B + ( 180 - A1 - B1 )      ,But A1=A ,B1=B             

          = A + B + 180 - A - B
          = 180
              
End Of Proof. #