已知(x+y)^2=324,(x-y)^2=16,求xy和x^2+y^2的值
问题描述：
已知(x+y)^2=324,(x-y)^2=16,求xy和x^2+y^2的值
答
(x+y)^2=324,==>X^2+Y^2+2XY=324 (1)
(x-y)^2=16 ==>X^2+Y^2-2XY=16 (2)
(1)+(2):2(X^2+Y^2)=340 ==>X^2+Y^2=170
(1)-(2):4XY=308 ===>XY=154