1.(1+y/x)/(1-y/x) 化简 2.已知x/y=3,求（x的平方+2xy-3y的平方）/（x的平方-xy+y得平方）

1.(1+y/x)/(1-y/x)
=1-(y/x)^2
=1-y^2/x^2
2
（x的平方+2xy-3y的平方）/（x的平方-xy+y得平方）
=(x+3y)(x-y)/(x^2-xy+y^2)
=(x+3y)(x-y)(x+y)/(x+y)(x^2-xy+y^2)
=(x+3y)(x-y)(x+y)/(x^3+y^3)
=y^3(x/y+3)(x/y-1)(x/y+1)/y^3(x^3/y^3+1)
=(3+3)(3-1)(3+1)/(3^3+1)
=48/28
=12/7