Sum of sequence: 1 * 3 + 3 * 5 + 5 * 7 +. + (2n-1) (2n + 1)
1*3+3*5+5*7+.+(2n-1)(2n+1)
=(4*1^2-1)+(4*2^2-1)+...+(4n^2-1)
=4*(1^2+2^2+...+n^2)-n
=4*n(n+1)(2n+1)/6 - n
=.
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