Hi ! I have some problem with this below !

Subjected to a square which has two isosceles triangles.

One of them is: a triangle whose base is one of the sides of the square root angles are 15.

The other is a triangle whose base is the side opposite the base of the triangle it first touches the apex of the triangle at the end of the first node.

Proved that the second triangle equilateral!

The prove must be geometric way , no trigonometric ! Please help me ! thanks all !