Because the quadrilateral internal angle and 360 degrees, ∠A=∠C, ∠B=∠D,

So ∠ A+∠ B = ∠ C+∠ D = ∠ A+∠ D = ∠ C+∠ B =180 degrees.

The sum of internal angles is equal to 180 degrees, so ABCD, AD∨BC, so the quadrilateral ABCD is a parallelogram.