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x+y=A、x-y=Bとする。
A+B=2x、A-B=2y
(A+B)(A-B)=4xy
(A+B)(A-B)/4=xy
よって、xy=((x+y)+(x-y))((x+y)-(x-y))/4
または、
A²=x²+2xy+y²、B²=x²-2xy+y²
A²-B²=4xy
(A²-B²)/4=xy
よって、xy=((x+y)²-(x-y)²)/4
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