Theorem 14

If two lines are intersected by a transversal

so that the consecutive interior angles are supplementary, then the lines are parallel.

_______________________1/2______________ l
              3 /4
            /
      ____________________5 / 6________________ m
     7  / 8
   /

1. / 3 is supplementary to / 5. 1. Given

2. / 7 is supplementary to / 5. 2. Linear pairs are supplementary.

3. / 3 = / 7 3. Supplements of the same angle are equal.

4. Line l is parallel to line m. 4. If corresponding angles are equal, then the

lines are parallel.

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