1 = 1
3 = 1 + 2 ·1
5 = 1 + 2 ·2
7 = 1 + 2 · 3
.............................
x = elementul n x = 1 + 2 ·( n -1)
1 + 2 + ... + [ 1 + 2 ·( n -1) ] = 225
[ 1 + 1 + 2 ·( n -1) ] · ( n -1) / 2 = 225
[ 2 + 2 · ( n -1) ] · n / 2 =225
2 · n · n / 2 = 225
n · n =225
n² = 225
n =15
x = 1 + 2 · ( n -1) = 1 + 2 · (15 -1) = 1 + 2 · 14 = 1 + 28 =29
1 + 2 + .... + 29 = 225
