in suma 3+7+11+15+....+119
sunt tot atatea nr daca adunam la fiecare 1, obtinem 4, 8, 12, 16, .....,120,
sunt tot atatea nr daca impartim fiecare la 4, obtinem 1, 2, 3, .....,30,
deci sunt 30 nr
S= 3+ 7+ 11+ 15+....+119
S=119+ 115+ 111+ 107+....+ 3
..............................................................
2S=122+122+122+122+.....+122
2S=122*cate sunt=122*30=3660
S=3660:2=1830