Given: m ∠3 = m ∠4 to prove: ∠1, ∠2 are supplementary

Statement                                           Reason

1) m<3 = m<4  :                                    (Given).

2) <2 and <3 are supplementary :     (Linear pair).

3) m<2 + m<3. =180. :                          Definition of supplementary angles.

4) m<2 + m<4. = 180. :                         Substitution.

5) m<1 = m<4. :                                    Opposite angles equal.

6) m<2 + m<1 = 180. :                          Substitution.

7) <1, <2 are supplementary :       Definition of consecutive exterior angles.