1+3+5+7+.+(2n-1)

问题描述:

1+3+5+7+.+(2n-1)

1+3+5+7+...+(2n-1) (等差数列)
1+3+5+7+...+(2n-1)
=(2n-1)*(1+2n-1)/2
=(2n-1)*n
=2n^2-n