已知(x+y)(x+y)=625/36 x-y=7/6 求xy的值？

(x+y)(x+y)=625/36
(x+y)^2=625/36
x+y＝±25/6

x-y＝7/6

当x+y＝25/6
x-y＝7/6时
x＝8/3
y＝3/2
xy＝4

当x+y＝-25/6
x-y＝7/6时
x＝-3/2
y＝-8/3
xy＝4

所以
xy＝4

∵(x+y)(x+y)=625/36
(x-y)^2=49/36
(x+y)^2-(x-y)^2=4xy=625/36-49/36=576/36
xy=576/36*4=4

4xy = (x+y)(x+y) -(x-y)(x-y)
= 625/36-49/36
= 16

xy =4

xy=4 过程是：（x+y）(x+y)=625/36 （x-y）(x-y)=(7/6)*(7/6)=49/36然后把两者都展开，拿上面的式子减下面的式子，最后得到4xy=576/36 最后得Xy=4。具体的过程就是这样。希望看的懂。

令A=(x+y)(x+y)=625/36
令B=(x-y)(x-y)=49/36
A-B=4xy=576/36=16
因此:xy=4

xy=4