a(n) = n*(25*n - 39)/2.

Terms

a(0) =0a(1) =-7a(2) =11a(3) =54a(4) =122a(5) =215a(6) =333a(7) =476a(8) =644a(9) =837a(10) =1055a(11) =1298a(12) =1566a(13) =1859a(14) =2177a(15) =2520a(16) =2888a(17) =3281a(18) =3699a(19) =4142a(20) =4610a(21) =5103a(22) =5621a(23) =6164a(24) =6732a(25) =7325a(26) =7943a(27) =8586a(28) =9254a(29) =9947