回復 3# P78961118 的帖子
\(\begin{align}
& \left( 1 \right)\quad \frac{1}{4}\times \frac{C_{2}^{12}}{C_{2}^{51}}+\frac{3}{4}\times \frac{C_{2}^{13}}{C_{2}^{51}} \\
& \\
& \left( 2 \right)\quad \frac{\frac{3}{4}\times \frac{C_{2}^{13}}{C_{2}^{51}}}{\frac{1}{4}\times \frac{C_{2}^{12}}{C_{2}^{51}}+\frac{3}{4}\times \frac{C_{2}^{13}}{C_{2}^{51}}} \\
\end{align}\)
