{(x+y)^2=9
(x-y)^2=5
{x^2+y^2+2xy=9 式1
x^2+y^2-2xy=5 式2
式1-式2：4xy＝4
xy＝1
(x+y)²=x²+2xy+y² = 9 ①
(x-y)²=x²-2xy+y² = 5 ②
① - ② =x²+2xy+y² - ( x²-2xy+y²) = 9 - 5
4xy = 4
xy= 1
(x+y}2-(x-y)2=9-5=4
x2+2xy+y2-x2+2xy-y2=4
4xy=4
xy=1
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