a) 1+3+5+7+9+11=
10 + 10 + 5 + 11 =
20 + 16 =
36
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b)1+3+5+7+...+19=
(19 - 1) : 2 - 1 = 18 : 2 + 1 = 10 termeni are suma
(19 + 1) × 10 : 2 =
20 × 10 : 2 =
20 × 5 =
100
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sau
2×1-1 + 2×2-1 + 2×3-1 + 2×4-1 +...+ 2×10-1 =
2 × ( 1 + 2 + 3 + 4 +...+ 10) - (1 + 1 +...+ 1 de 10 ori)
2 × 10 × 11 : 2 - 1 × 10 =
10 × 11 - 10 =
10 × ( 11 - 1) =
10 × 10 =
100
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