1+2+3+…+2002+2003+2002+…+3+2+1 S1=1+2+........+2002 +2003 S2=2002+2001+..+1 S1+S2=1+2002+2+2001+...+2002+1+ 2003 =2003+2003+..+2003+2003 =2003*2003 =2003^2
