Geometry Problem 1138: Square,
Perpendicular, Angle, 90 Degrees, Triangle, Congruence. Level: High School, SAT Prep,
Honors Geometry, College, Math Education

In a square ABCD (figure below), E
is on AB extended and F on AD extended so that BE = DF. If AG is
perpendicular to DE, prove that the measure of angle CGF is 90
degrees.