该等差数列的首项是1,公差是3,所以通项公式是an = 1 + 3(n - 1) = 3n - 2, 所以前n项和为Sn = (1 + 3n - 2)n/2 = (3n-1)n/2,前一百项和为（3*100 - 1)*100 / 2 = 14950
