急,作业要写的,快,十分钟悬赏五十

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(-1*1/2）+(-1/2*1/3)+(-1/3*1/4)…+(-1/2012*1/2013）
=（1/2-1）+(1/3-1/2)+(1/4-1/3).+(1/2013-1/2012)
=-1+1/2-1/2+1/3-1/3+1/4-1/4.-1/2012+1/2013
=-1+1/2013
=-2012/2013