x-y=3,(x+y)(x+y)=25,求xy/xx+yy的值?

问题描述:

x-y=3,(x+y)(x+y)=25,求xy/xx+yy的值?

x-y=3
平方
x²-2xy+y²=9
x²+2xy+y²=(x+y)²=25
相加
2(x²+y²)=34
x²+y²=17
2xy=x²+y²-9=8
xy=4
所以原式=4/17