1 + 3 + ... + (2n - 1) = 1 + 2 + ... + (2n - 1) - 2 - 4 - 6 - ... - (2n - 2) =
= (1 + ... + (2n - 1)) - 2 * (1 + 2 + 3 + ... + (n - 1))
Так как 1 + ... + n = n * (n + 1)/2, то
1 + 3 + ... + (2n - 1) = (2n - 1) * (2n)/2 - 2 * (n - 1) * n/2