求助！！求一道数学题的解，急用，在线等，直到的速度

{（x-2y）(x-y)+(x²-y²)-(-x-y)²}÷5x=
{（x-2y）(x-y)+[(x-y)-(x+y)]*(x+y)}÷5x=
{（x-2y）(x-y)-2y*(x+y)}÷5x=
{（x²-3xy+2y²)-2xy-y²)}÷5x=
(x²-5xy)÷5x=
x(x-5y)÷5x=
(x-5y)÷5=
5÷5=1

{（x-2y）(x-y)+(x²-y²)-(-x-y)²}÷5x=
{（x-2y）(x-y)+[(x-y)-(x+y)]*(x+y)}÷5x=
{（x-2y）(x-y)-2y*(x+y)}÷5x=
{（x²-3xy+2y²)-2xy-y²)}÷5x=
(x²-5xy)÷5x=
x-5y=
5

先将大括号内后两项提取提取公因式（x+y），再就好做了！

答案：1

因为x-5y=5，所以x=5y+5
代入
{（x-2y）(x-y)+(x²-y²)-(-x-y)²}÷5x
={(3y+5)(4y+5)-2y(y+5)-2y^2}/5(5y+5)
=(12y^2+35y+25-10y^2-10y-2y^2)/25(y+1)
=(25y+25)/25(y+1)
=25(y+1)/25(y+1)
=1

1