ABCD is a quadrilateral in which AB || DC and AD || BC. A line MN, parallel to CD, is drawn meets the midpoint M of the side BC which meets AD at N. What can you say about the diagonal AC?
Line MN bisects AC.
Since AB, CD and MN are all parallel lines, we can think of AD, AC and BC as transversals on the parallel lines.
∴ By equal intercept theorem, AN = ND and AO = OC.
⇒ Line MN bisects diagonal AC.