If an=an−1+2na_n = a_{n-1} + 2nan=an−1+2n, what is ana_nan expressed as a summation?
an=a0+∑k=1n2ka_n = a_0 + \sum_{k=1}^n 2kan=a0+∑k=1n2k
an=a0+∑k=0nka_n = a_0 + \sum_{k=0}^n kan=a0+∑k=0nk
an=a0+2na_n = a_0 + 2^nan=a0+2n
an=a0+∑k=1nk2a_n = a_0 + \sum_{k=1}^n k^2an=a0+∑k=1nk2