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