Iven: segment ac is perpendicular to segment bd prove: δacb ~ δecd reflect δecd over segment ac. this establishes that then, this establishes that ∠e'd'c' ≅ ∠abc. therefore, δacb ~ δecd by the aa similarity postulate. ∠abc ≅ ∠e'd'c'; translate point e' to point a ∠acb ≅ ∠e'c'd'; translate point e' to point b ∠acb ≅ ∠e'c'd'; translate point d' to point b ∠abc ≅ ∠e'd'c'; translate point d' to point a