Question

已知(x+y)^2=8,(x-y)^2=5,求4xy与x^2+y^2的ŀ

Answer 1

(x+y)²=8
x²+2xy+y²=8①

(x-y)²=5
x²-2xy+y²=5
②
①+②得：2x²+2y²=13

x²+y²=13/2

①-②
4xy=3

Answer 2

(x+y)²=8 x²+2xy+y²=8
(x-y)²=5 x²-2xy+y²=5
两式相加得：2x²+2y²=13
x²+y²=13/2
两式相减得：4xy=3