已知(x+y)(x+y)=25,(x-y)(x-y)=9求xy与xx+yy的值

问题描述:

已知(x+y)(x+y)=25,(x-y)(x-y)=9求xy与xx+yy的值

(x+y)(x+y)=25
x^2+2xy+y^2=25……(1)
(x-y)(x-y)=9
x^2-2xy+y^2=9……(2)
(1)+(2)得:
2x^2+2y^2=34
x^2+y^2=17
(1)-(2)得:
4xy=16
xy=4