Chứng minh rằng: A = 77^(n+1) - 77^n chia hết cho 76

CMR: $$\begin{gathered} \mathrm{a}) \mathrm{A} = {77}^{\mathrm{n}+1} - {77}^{\mathrm{n}} ⋮ 76 \\ \mathrm{b}) \mathrm{B} = {\mathrm{n}}^2 .( \mathrm{n} + 1) + 2\mathrm{n}.\mathrm{n}(\mathrm{n} + 1) ⋮ 6 ( \mathrm{n} \in \mathrm{Z}) \end{gathered}$$