(1+2+3+4+5+6+7+8+...+2012+2013+2012+...+3+2+1)/2013
=[(1+2+3+4+5+6+7+8+...+2012+2013)+(2012+...+3+2+1)]/2013
=[(1+2013)*2013*(1/2) + (2012+1)*2012*(1/2)]/2013
=[ 2014*2013*(1/2) + 2013*2012*(1/2)]/2013
=[2014*(1/2) + 2012(1/2)]
= 2013