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15+16+17+.........+988

Răspuns :

1+2+3+4+...+15+16+17 +...+988 = S₁₋₁₄ + S₁₅₋₉₈₈ = S₁₋₉₈₈

S₁₋₁₄ = 14*15:2=7*15= 105
S₁₋₉₈₈ = 988*989:2= 488566
S₁₅₋₉₈₈= 488566 - 105= 488461

15+16+17+...+988 = 488461