It is given that and are right angles, and . since they contain right angles, δabr and δacr are right triangles. the right triangles share hypotenuse , and reflexive property justifies that . since and , the transitive property justifies . now, the hypotenuse and leg of right δabr is congruent to the hypotenuse and the leg of right δacr, so by the hl congruence postulate. therefore, by cpctc, and bisects by the definition of bisector.