25(m+n)²－4(m－n）²
=25(m²＋2mn+n²)－4(m²－2mn＋n²）
=25m²＋50mn+25n²－4m²＋8mn－4n²
=﹙25m²－4m²﹚＋﹙50mn＋8mn﹚+﹙25n²－4n²﹚
=21m²＋58mn＋21n
=(3m+7n)(7m+3n)