Given: <1 and <2 are supplementary angles Prove: l || m

Question

Given: <1 and <2 are supplementary angles

Prove: l || m

 

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STATEMENT                                                               REASON
1.∠1 and ∠2 are supplementary angles                        1. Given
2. m∠1 + m∠2 = 180°                                                2.______
3. ∠1 and ∠3 are supplementary angles                       3. Exterior sides in opposite rays
4.________                                                               4._________
5. m∠1 + m∠2 = m∠1 + m∠3                                      5._________
6._________                                                              6._________
7. l || m                                                                    7.__________

Answer 1

 

I think you are trying to show m∠2 = m∠3 as that implies  l || m   (if alternate interior angles are equal then the lines bounding them are parallel).    Step 5 is the bridge, as you can subtract m∠1 from both sides, no?