a(n) = 5*n*(5*n + 1)/2.

Terms

a(0) =0a(1) =15a(2) =55a(3) =120a(4) =210a(5) =325a(6) =465a(7) =630a(8) =820a(9) =1035a(10) =1275a(11) =1540a(12) =1830a(13) =2145a(14) =2485a(15) =2850a(16) =3240a(17) =3655a(18) =4095a(19) =4560a(20) =5050a(21) =5565a(22) =6105a(23) =6670a(24) =7260a(25) =7875a(26) =8515a(27) =9180a(28) =9870a(29) =10585