Question

已知 (x+y)^2=4,(x-y)^2=3,试求:(1)x^2+y^2的值;(2)xy的值.

Answer 1

x²+2xy+y²=4

x²-2xy+y²=3

两式相加

2(x²+y²)=7

x²+y²=7/2

两式想减

4xy=1

xy=1/4