原式=[3(x+Y)³-5(x+y)²-2（x+y）]/(x+y)
=3(x+y)²-5(x+y)-2
=3(x+y)(x+y-5)-2
=3x²+6xy＋3y²－5x－5y－2
有什么不懂的可以继续追问,