劳力士走时误差:约分 题目

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-(x+y)(x-y)^2 /(y+x)(y^2-x^2)

-(x+y)(x-y)^2 /(y+x)(y^2-x^2)
=(x-y)^2/(x-y)(x+y)
=(x-y)/(x+y)

X-Y