We have $y_{k}=y_{k-1}+k^2$, so \begin{eqnarray*} y_{k}=k^2+(k-1)^2+ \cdots+ 1 =\sum_{i=1}^{k} i^2 =\frac{n(n+1)(2n+1)}{6}. \end{eqnarray*}
We have $y_{k}=y_{k-1}+k^2$, so \begin{eqnarray*} y_{k}=k^2+(k-1)^2+ \cdots+ 1 =\sum_{i=1}^{k} i^2 =\frac{n(n+1)(2n+1)}{6}. \end{eqnarray*}