Question

1+3+5+.................+3999 cat face pls

Answer 1

1 + 3 + 5 + ... + 2n-1 = $n^{2}$
3999 = 2n-1 => 4000 = 2n => n = 2000
S= $n^{2} = 2000^2 = 4,000,000$