Sections AB and CD of parallel straight lines are enclosed between parallel planes. Find AB if CD = 3cm.

Given:
AB || DC
DC = 3cm
To find:
AB =?
Decision:
1) First, let’s connect points A and C on one plane and B and D on another plane with straight lines.
2) AC is parallel to BD, since they lie in parallel planes and the distance from A to plane “b” is equal to the distance from C to plane “b”, respectively, for the same reason, points B and D. Quadrangle ABCD – parallelogram. Since in the parallelogram AB = CD, and CD = 3 cm, then AB = 3 cm.
Answer: AB = 3 cm.