Using two basic knowledge, one is that the sum of the internal angles of a triangle is 180 degrees; Second, the vertex angles are equal.

For the convenience of writing, we call the unlabeled diagonal corners Angle 5 and Angle 6 respectively, where Angle 1, Angle 2 and Angle 5 form a triangle; Wherein Angle 3, Angle 4 and Angle 6 form another triangle;

Because: angle 1+ angle 2+ angle 5= 180.

Angle 3+ Angle 4+ Angle 6= 180

So: angle 1+ angle 2+ angle 5= angle 3+ angle 4+ angle 6.

Because: Angle 5= Angle 6 (equal to the vertex angle)

So: angle 1+ angle 2= angle 3+ angle 4.