数学题已知

来源：百度知道 编辑：UC知道 时间：2024/07/07 14:37:01
已知
1 1
-------- - ---------- =5
x y

求 3x+5xy-3y
------------------ =?
x-3xy-y
1 1
-------- - ---------- =5
x y

求 3x+5xy-3y
------------------ =?
x-3xy-y

已知
1/x-1/y=5,
2边都乘以xy.得:
y-x=5xy

求 (3x+5xy-3y)/(x-3xy-y)
=[-3(y-x)+5xy]/[-(y-x)-3xy]
=-10xy/(-8xy)
=5/4

看不明白

什么呀

1/x-1/y=5
(y-x)/xy=5
y-x=5xy
x-y=-5xy

(3x+5xy-3y)/(x-3xy-y)
=[3(x-y)+5xy]/[(x-y)-3xy]
=(-15xy+5xy)/(-5xy-3xy)
=-10xy/(-8xy)
=5/4

1/x-1/y=5
有：
x-y=-5xy
代入有：
3(x-y)+5xy/(x-y)-3xy
=-10xy/-8xy
=5/4

1.25 x-y=a xy=b即可