>338
ｎ以下の和をS(n)とおくと、
　S(1) = 1/6 + 5/6 = 1.
　S(n) = S(n-1) + (6n-5)/6 + (6n-1)/6 = S(n-1) + 2n -1.
　∴ S(n) = n^2.
　S(10) = 10^2 = 100.