Solve an=an−1+na_n = a_{n-1} + nan=an−1+n with a0=0a_0 = 0a0=0.
an=n2a_n = n^2an=n2
an=n(n+1)/2a_n = n(n+1)/2an=n(n+1)/2
an=2na_n = 2^nan=2n
an=n!a_n = n!an=n!