eq 1 DBC + 90 + DCB = 180 eq 2 DAC + 90 + ACD = 180 eq 1 DBC + DCB = 90 eq2 DAC + ACD = 90 eq 3 DCB + ACD = 90 therefore, DCB = 90 - ACD substitute into eq 1: DBC + 90 - ACD = 90 --> DBC = ACD