A trapezium is a quadrilateral with at least one pair of parallel sides. In one such trapezium the two parallel sides measure 14 cm and 22 cm. What is the length of the segment joining the midpoints of the two non-parallel sides?

The segment joining the midpoints of the non-parallel sides is the mid-segment, and it equals the average of the two parallel sides. So its length is (14 + 22)/2 = 18 cm. The value 8 cm is their difference and 36 cm their sum, neither of which is the mid-segment.