Question

已知(x+y)2=20,(x-y)2=40,求：（1）x2+y2的值；（2）xy的值.

Answer 1

x^2+2xy+y^2=20

x^2-2xy=y^2=40

两式想加 2x^2+2Y^2=20+40=60

X^2+Y^2=30

两式相减

4xy＝20-40＝-20

xy＝-5