s=16+21+26+31+...+111
aflam cati termeni are sirul:
(111 -16) : 5 + 1 = 95 : 5 + 1 = 20
S = (111 + 16) × 20 : 2
S = 127 × 10
S = 1 270
sau
S = 5×1+11 + 5×2+11 + 5×3+11 +5×4+11+...+5×20+11
S = 5×(1+2+3+4+...+20) + 11+11+...+11 de 20 ori
S = 5 × 20 × 21 : 2 + 11 × 20
S = 5 × 10 × 21 + 220
S = 50 × 21 + 220
S = 1050 + 220
S = 1 270
