第一种：xy(xy+4)-2(x+y)(x²-xy+y²)
=x²y²＋4xy-2(x³-x²y+xy²+x²y-xy²+y³)
=x²y²＋4xy-2x³-2y³
=(x²-2y)(-2x+y²)
第二种：
xy(xy+4)-2(x+y)(x²-xy+y²)
=x²y²＋4xy-2(x+y）[（x＋y）²-3xy]
=x²y²＋4xy-2(x+y)³+6xy(x＋y)
=x²y²＋4xy-2(x³+3x²y+3xy²+y³)+6x²y+6xy²
=x²y²＋4xy-2x³-2y³
=(x²-2y)(-2x+y²)