（1/x+1/y）÷（1/x^2-1/y^2)
= (x+y)/(xy) ÷ (y^2-x^2)/(x^2y^2)
= (x+y)/(xy) * (x^2y^2)/(y^2-x^2)
= (x+y)/(xy) * (xy)^2/{-(x+y)(x-y)}
= -xy/(x-y)
（1/x+1/y）÷（1/x^2-1/y^2)
=（1/x+1/y）÷[(1/x+1/y)(1/x-1/y)]
=1÷(1/x-1/y)
=1÷[(y-x)/xy]
=xy/(y-x)
xy/(y-x)
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