a(n) = (25*n^2 - 15*n + 2)/2.

Terms

a(0) =1a(1) =6a(2) =36a(3) =91a(4) =171a(5) =276a(6) =406a(7) =561a(8) =741a(9) =946a(10) =1176a(11) =1431a(12) =1711a(13) =2016a(14) =2346a(15) =2701a(16) =3081a(17) =3486a(18) =3916a(19) =4371a(20) =4851a(21) =5356a(22) =5886a(23) =6441a(24) =7021a(25) =7626a(26) =8256a(27) =8911a(28) =9591a(29) =10296