You may only use elementary geometry, such as the fact that the angles of a triangle add up to 180 degrees and the basic congruent triangle rules (side-angle-side, etc.).

So anyway, this is the origin, and then the last side, y equals zero, x So, somehow, in the change of variables, this square becomes this triangle.

Subdivide the diagonal section at the back-left side of the triangle with one diagonal-back parting.