Objective:

To show that the figure obtained by joining the mid-points of consecutive sides of the quadrilateral is a parallelogram.

Parallelogram:

1. A parallelogram is a simple quadrilateral with two pairs of parallel sides.
2. The opposite or facing sides of a parallelogram are of equal length.

Proof:

In quadrilateral ABCD points P, Q, R, S are midpoints of side AB, BC, CD and AD respectively.

To prove :

PS || QR  and SR || PQ. i.e. Quadrilateral PQRS is a parallelogram

Proof:

1. Draw diagonal BD.
2. As PS is the midsegment of ▲ ABD, we can say that PS || BD.
3. As QR is the midsegment of ▲ BCD, we can say that QR || BD.
4. ∵ PS || BD and QR || BD by transitivity, we can say that PS || QR.
5. Now draw diagonal AC.
6. As SR is the midsegment of ▲ ACD, we can say that SR || AC.
7. As PQ is the midsegment of ▲ ABC, we can say that PQ || AC.
8. ∵ SR || AC and PQ || AC by transitivity, we can say that SR || PQ.
9. ∵ PS || QR and SR || PQ, ∴ quadrilateral PQRS is a parallelogram (by definition).