Answer: The slopes of two parallel lines are equal.
Step-by-step explanation: Let
and
be the slopes of lines AB and MN respectively. We know that if the slopes of two parallel lines are equal, then the lines must be parallel. Thus, if we get
, then AB is parallel to MN. Also, if we already have M and N as the mid-points of AC and BC, the with the help of slopes of AB and MN, we arrive at our proof that MN is the mid-segment of Triangle ABC. Please see the figure for details.