已知1/x+1/y=1/2，求代数式3x-5xy+3y/-x+3xy-y

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

特殊值法。。X=Y=4满足条件
那么嗲如就可以了！
3x-5xy+3y/-x+3xy-y
=12-80-3+48-4
=-27

1/x+1/y=1/2 得xy=2(x+y)
则3x-5xy+3y/-x+3xy-y=[3x-10(x+y)+3y]/-x+6(x+y)-y= -7/5