将(x+y)^2=18展开得x^2+y^2+2xy=18 ①
将(x-y)^2=6展开得x^2+y^2-2xy=6 ②
①+②得x^2+y^2=12；①-②得4xy=12,xy=3 .
x²+y²
=[(x+y)²+(x-y)²]/2
=(18+6)/2
=12
xy
=[(x+y)²-(x-y)²]/4
=(18-6)/4
=3
（x+y)^2-(x-y)^2=4xy=12 xy=3
（x+y)^2+(x-y)^2=2x^2+2y^2=24 x^2+y^2=12
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