1/x+1/y=1/2
(x+y)/xy=1/2
xy=2(x+y)
所以(3x-5xy+3y)/(-x+3xy-y)
=[3(x+y)-5*2(x+y)]/[-(x+y)+3*2(x+y)]
=-7(x+y)/[5(x+y)]
=-7/5