xy/x+y=3求2x-3xy+2y/3xy-x-y的值

xy/(x+y)=3,
xy=3(x+y),
(2x-3xy+2y)/(3xy-x-y)
=[2(x+y)-3xy]/[3xy-(x+y)]
=[2(x+y)-3*3(x+y)]/[3*3(x+y)-(xy)]
=-7(x+y)/8(x+y)
=-7/8.

因为xy/x+y=3 所以 xy=3(x+y)
2x-3xy+2y/3xy-x-y=(2x-9x-9y+2y)/(9x+9y-x-y)
=-7(x+y)/8(x+y)
=-7/8

xy=3(x+y)
带入 2x-3xy+2y=2(x+y)-3*3(x+y)=-7(x+y)
3xy-x-y=3xy-(x+y)=8(x+y)
所以 要求的值为-7/8