Please help me how to display this sum of series?

Question

0+3+8+15..................n

Answer 1

hint: a_n = n * (n + 2)

a_0 = 0 * (0 + 2) = 0 * 2 = 0
a_1 = 1 * (1 + 2) = 1 * 3 = 3
a_2 = 2 * (2 + 2) = 2 * 4 = 8
a_3 = 3 * (3 + 2) = 3 * 5 = 15
and so on

total = sum([i * (i + 2) for i in range(n)]

Answer 2

I know this: 1+2+3+4+. . . +n = sumN = n*(n+1)/2

for that perhaps change some terms