## Elementary Geometry for College Students (5th Edition)

Sum of all the angles of a quadrilateral = 360 BA $\bot$AD AND BC $\bot$DC $\angle$A = $\angle$C =90 *....(1) $\angle$BAD +$\angle$ADC+$\angle$DCB+$\angle$CBA =360 $\angle$A +$\angle$B+$\angle$C+$\angle$D =360 90 + $\angle$B + 90 + $\angle$D = 360 ...USING (1) $\angle$B + $\angle$D = 360- 180 $\angle$B + $\angle$D = 180 Hence proved