Question

(a+b-c-d)²

Answer 1

（a十b一c一d）²
=（a十b）²-2（a十b）（C十d）+（c十d）²
=a²十b²十2ab一2ac一2ad一2bc一2bd十C²+2cd+d²
=a²十b²十c²十d²十2ab一2ac一2ad一2bc一2bd十2cd

Answer 2

(a+b-c-d)²
=[(a+b)-(c+d)]²
=(a+b)²-2(a+b)(c+d)+(c+d)²
=a²+2ab+b²-2ac-2ad-2bc-2bd+c²+2cd+d²