∵a²+b²+c²+d²=ab+bd+bc+cd∴2a²+2b²+2c²+2d²-2ab-2bd-2bc-2cd=0∴(a-b)²+(b-c)²+(c-d)²+(d-a)²=0∴a-b=0 b-c=0 c-d=0 d-a=0∴a=b=c=d∴四边形为菱形。亲，满意请及时采纳啊
