y分之x=2分之1,
y=2x
x的平方+2xy+y的平方分之x的平方y-x的三次方除以x-y分之x乘x的平方-2xy+y的平方分之x+y
=x^2y/(x^2+2xy+y^2) - x^3/ [x/(x-y)] *(x+y)/(x^2-2xy+y^2)
=x^2y/(x+y)^2 - x^3 * [(x-y)x] *(x+y)/(x-y)^2
=x^2y/(x+y)^2 - x^2(x+y)/(x-y)
=x^2*2x/(x+2x)^2 - x^2(x+2x)/(x-2x)
=2x^3/9x^2 - 3x^3/(-x)
=2x/9 +3x^2