(a－b＋c－d)(a＋b－c－d)
＝(a－d－b＋c)(a－d＋b－c)
＝{(a－d)－(b－c)}{(a－d)＋(b－c)}
●(a－d)＝Ｐ，(b－c)＝Ｑと置いて
＝{Ｐ－Ｑ}{Ｐ＋Ｑ}
＝Ｐ²－Ｑ²
●Ｐ＝(a－d)，Ｑ＝(b－c)と戻して
＝(a－d)²－(b－c)²
＝(a²－2ad＋d²)－(b²－2bc＋c²)
＝a²－2ad＋d²－b²＋2bc－c²
＝a²－b²－c²＋d²－2ad＋2bc
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