Given: Rectangle ABCD, CF perpendicular to BD

Prove: AD / EC = AB / ED

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