回復 19# EZWrookie 的帖子
誤版第8題
您只差一步
\begin{align}
& \frac{k}{k!+\left( k-1 \right)!+\left( k-2 \right)!} \\
& =\frac{\frac{k}{\left( k-1 \right)!}}{k+1+\frac{1}{k-1}} \\
& =\frac{\frac{k}{\left( k-1 \right)!}}{\frac{{{k}^{2}}}{k-1}} \\
& =\frac{k-1}{k!} \\
& =\frac{1}{\left( k-1 \right)!}-\frac{1}{k!} \\
\end{align}