As shown in the figure, in quadrilateral AB=CD, AB = CD, M, N and P are the midpoint of AD, BC and AC respectively. Please judge the shape of PMN angle and explain the reason.

△PMN is an isosceles triangle. The reason is:

∫M and p are the midpoint of AD and AC respectively,

∴MP=？ CD.

∫N and p are the midpoint of BC and AC respectively,

∴NP=？ AB。

AB = CD，

∴MP=NP.

△ PMN is an isosceles triangle.