(-1×1/2)+(-1/2×1/3)+(-1/3×1/4)+...+(-1/2004×1/2005)

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(-1×1/2)+(-1/2×1/3)+(-1/3×1/4)+...+(-1/2004×1/2005)
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(-1×1/2)+(-1/2×1/3)+(-1/3×1/4)+...+(-1/2004×1/2005)
(-1×1/2)+(-1/2×1/3)+(-1/3×1/4)+...+(-1/2004×1/2005)

(-1×1/2)+(-1/2×1/3)+(-1/3×1/4)+...+(-1/2004×1/2005)
原式
=(-1的2004次方)*(1×1/2)+(1/2×1/3)+(1/3×1/4)+...+(1/2004×1/2005)
=(1×1/2)+(1/2×1/3)+(1/3×1/4)+...+(1/2004×1/2005)
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/2004-1/2005)
=1-1/2005
=2004/2005

-1

由式子 1/n-1/(n+1)=1/n(n+1)
该式可以将每个括号里的 - 提取出来
就成了-[1×1/2+1/2×1/3...+1/2004×1/2005]=-(1-1/2005)=-2004/2005