AA similarity

Perpendicular lines form right angles

AD / EC = AB / ED

Definition

∠DEC = ∠A

Rectangle ABCD, CF ⟂ BD

∠ABD = ∠CDE

Given

Definition

Parallel lines form equal alternate interior angles

∠A is a right angle

ABCD is a parallelogram

AB ǁ CD

Definition

△DAB ~ △CED

∠DEC is a right angle

All right angles are equal

All rectangles are parallelograms