解
原式
=2(x-y)²-3(x-y)³-(x-y)²+2(x-y)³-6(x-y)²
=(2-1-6)(x-y)²+(2-3)(x-y)³
=-5(x-y)²-(x-y)³-6(x-y)² 为什么能这样写∵6(y-x)²=6[-(x-y)]²=6(x-y)²偶次方是正的