(2)已知(x+y)^2=31,(x-y)^2=19,求X^2+y^2及xy的值.

问题描述:

(2)已知(x+y)^2=31,(x-y)^2=19,求X^2+y^2及xy的值.

x2+y2+2xy=31x2+y2-2xy=19两式相加得:2x2+2y2=31+19=50x2+y2=25两式相减得:4xy=31-19=12xy=3