a的3次方+b的三次方+15ab
=(a＋b)(a²－ab＋b²)+15ab
=5(a²－ab＋b²)+15ab(代人a+b=5)
=5a²－5ab＋5b²＋15ab
=5a²＋10ab＋5b²
=5(a²＋2ab＋b²)
=5(a＋b)²(代人a+b=5)
=5×5²
=125