Let ABCD be a parallelogram with acute angle at A. Let E be the foot of the perpendicular from C to the
line AB and let F be the foot of the perpendicular from C to the line AD.

Let ABCD be a parallelogram with acute angle at A. Let E be the foot of the perpendicular from C to the
line AB and let F be the foot of the perpendicular from C to the line AD.