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Calculeaza suma S=2+5+8+..............+254.

Răspuns :

S=2+5+8+..............+254
S = 3×0+2+3×1+2+3×2+2+...+3×84+2
S = 3×(0+1+2+...84) + 2+2+2+...+2 de 85 ori
S = 3×84×85:2 + 2×85
S = 3×42×85 + 170
S = 10710 + 170
S = 10 880