∴AM=CN and am∨cn

∴ Quadrilateral ANCM is a parallelogram

∴AN∥CM

Similarly, BM sigma dn

∴ Quadrilateral PNQM is a parallelogram

∫AM = AD/2 = 2AB/2 = AB，AD∨BC。

∴∠ 1=∠2,∠ 1=∠3

∴∠2=∠3

And AB=BN,

∴BP vertical safety,

That is, ∠NPM is a right angle,

So the parallelogram PNQM is a rectangle.