1 + 3 + 5 + ........ + 213 = 107 x 107 = 11 449
2 n - 1 = 213
2 n = 213 + 1
n = 214 : 2
n = 107
Formula: 1 + 3 + 5 + ....... + ( 2 n - 1 ) = n x n = n²
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sau:
1 + 3 + 5 + ........ + 213 =
→ stabilim cati termeni are suma cu ratia 2 : ( 213 - 1 ) : 2 + 1 = 212 : 2 + 1 = 106+ 1 = 107 termeni are suma
= 107 × ( 1 + 213 ) / 2 = 107 × 214 / 2 = 107 x 107 = 11 449
