x3次方y-2x2y2+xy3
=xy(x²-2xy+y²)
=xy（x-y）²
=3x3²
=27
如果本题有什么不明白可以追问,=xy(x2-2xy+y2)=xy(x-y)2当x-y=-1，xy=3时原式=3x3次方y-2x2y2+xy3=xy(x²-2xy+y²)=xy（x-y）²=3x1²=3a4-2a2b2+b4=?