S=1+ 2+....... +2002
S=2002+2001+.......1
Am scris suma crescator si descrescator. Suma fiecarui termen este 2003 si sunt in total 2002 termeni
2S=2002*2003 deci S=2002*2003/2
a=2003+2x2002*2003/2=2003+2002*2003=2003(1+2002)=2003*2003 deci este patrat perfect