（-1X1/2）+（-1/2X1/3）+（-1/3X1/4）+。。。+（-1/2009X1/2010）=-(1-1/2+1/2-1/3+1/3-1/4+.......+1/2009-1/2010)=-(1-1/2010)=-2009/2010
