原式=1-1/2+1/2-1/3+1/3-1/4+.1/2008-1/2009
=1-1/2009
=2008/2009
就是1/（n-1)*n=1/(n-1)-1/n
1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+......1/2008x2009
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