[Real Matrik]

#30
$$b_n=\sum^{n}_{k=1}(k^2+k)=\sum^{n}_{k=1}k^2+\sum^{n}_{k=1}k=\frac{n(n+1)(2n+1)}{6}+\frac{n(n+1)}{2}=\frac{n(n+1)(n+2)}{3}$$