At this point, the detective was concerned that the parallelogram postulates could be circumstantial evidence. In other words, the postulates were true for a few specific parallelograms, but did they hold true in general? Because he was an excellent detective, Reese knew he needed proof. First, he wanted to prove that opposite angles in a parallelogram are congruent. To do so, he used the steps listed in the table below. Click each step in the table to examine the proof.

	In parallelogram EFGH, prove ∠E $\mathsf{ \cong }$ ∠G.
	Draw diagonal FH to create △EFH and △FHG.
	By the definition of a parallelogram, EF $\mathsf{ \cong }$ HG and FG $\mathsf{ \cong }$ EH. Also, FH $\mathsf{ \cong }$ FH by the Reflexive Property of Congruence. By SSS, △EFH $\mathsf{ \cong }$ △FHG.
	By CPCTC, ∠E $\mathsf{ \cong }$ ∠G.

Detective Reese was now convinced that opposite angles in a parallelogram are congruent.