Given: Rectangle ABCD, CF perpendicular to BD

Prove: AD / EC = AB / ED

Definition
<ABD = <CDE
Parallel lines form equal alternate interior angles
Definition
AB || CD
ABCD is a parallelogram
Rectangle ABCD, CF perpendicular to BD
Given
<A is a right angle
Definition
Triangle DAB ~ Triangle CED
<DEC = <A
AD / EC = AB / ED
AA similarity
Perpendicular lines form right angles
<DEC is a right angle
All right angles are equal
All rectangles are parallelograms