因:AB=AC 所以:角C=角ABC 角A=180度-2角C ································⑴
因:DA=DB 所以:角A=角ABD 角ADB=180-2角A································⑵
因:BD=BC 所以:角C=角BDC 角DBC=180度-2角C····························⑶
由:⑴、⑶ 得:角A=角DBC
又因:角A=角ABD
所以:角ABC=2角A 角C=2角A
代入⑴得:
角A=180度-4角A
角A=36度
角C=角ABC=36度x2=72度