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calculeaza suma 11+22+33+....+198

Răspuns :

11+22+33+...+198 =
11×1 + 11×2 + 11×3 +...+ 11×18 =
11 × (1+2+3+...+18) =
11 × 18×19:2 =
11×9×19 =
99×19 = 1881