设x方y方-20xy+x方+y方+81=0,求x.y的值
问题描述:
设x方y方-20xy+x方+y方+81=0,求x.y的值
答
x²y²-20xy+x²+y²+81
=x²y²-18xy+81+x²-2xy+y²
=(xy-9)²+(x-y)²
=0
∴xy-9=0并且x-y=0
∴x=y=3或-3