若x+y=2,x^2+y^2=15,求xy的值

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若x+y=2,x^2+y^2=15,求xy的值
若(x+y)^2=7,(x-y)^2=5,求xy的值

x+y=2
(x+y)²=2²
x²+2xy+y²=4
15+2xy=4
xy= -11/2
再问：第几题？第二题呢？
再答： (x+y)²=7 x²+2xy+y²=7………………………………① (x-y)²=5 x²-2xy+y²=5………………………………② ①+②得：2x²+2y²=12，即x²+y²=6……③ ③代入①得，xy = -1/2

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