[2(x-y)³-8(x-y)²(x+y)+6y(x-y)²]÷2（x-y）².

[2(x-y)³-8(x-y)²(x+y)+6y(x-y)²]÷2（x-y）².
[2(x-y)³-8(x-y)²(x+y)+6y(x-y)²]÷2（x-y）².

[2(x-y)³-8(x-y)²(x+y)+6y(x-y)²]÷2（x-y）².
原式=2(x-y)²[(x-y)-4(x+y)+3y]÷2(x-y)²
=x-y-4x-4y+3y
=-3x-2y