He proof that is shown. Given: ΔMNQ is isosceles with base , and and bisect each other at S.
Prove:

Square M N Q R is shown with point S in the middle. Lines are drawn from each point of the square to point S to form 4 triangles.

We know that ΔMNQ is isosceles with base . So, by the definition of isosceles triangle. The base angles of the isosceles triangle, and , are congruent by the isosceles triangle theorem. It is also given that and bisect each other at S. Segments are therefore congruent by the definition of bisector. Thus, by SAS.

NS and QS
NS and RS
MS and RS
MS and QS