若1/x+2/y=5,求代数式2x+xy+y/8x-3xy+4y的值.

1/x+2/y=5,
则y/(xy)+2x/(xy)=5
(2x+y)/(xy)=5
2x+y=5xy
则2x+xy+y/8x-3xy+4y
=2x-y+xy/4(2x-y)-3xy
=5xy+xy/20xy-3xy
=6xy/17xy
=6/17

1/x+2/y=5
(2x+y)/xy=5
2x+y=5xy
所以原式=[2(2x+y)+xy]/[4(2x+y)-3xy]
=[2(5xy)+xy]/[4(5xy)-3xy]
=11xy/17xy
=11/17

1/x+2/y=5
(y+2x)/xy=5
y+2x=5xy
2x+xy+y/8x-3xy+4y
=(2x+y+xy)/[4(2x+y)-3xy]
=(5xy+xy)/[4*5xy-3xy]
=6xy/17xy
=6/17

2x+xy+y/8x-3xy+4y，
分子分母都除以xy得
（1/x+2/y+1)/[4(1/x+2/y)-3]
=(5+1)/[4*5-3]
=6/17.