Let a
$$
\begin{aligned}
t a=1+\frac{{ }^2 C_2}{3!}+\frac{{ }^3 C_2}{4!}+\frac{{ }^4 C_2}{5!}+\cdots, & \\
& \quad b=1+\frac{{ }^1 C_0+{ }^1 C_1}{1!}+\frac{{ }^2 C_0+{ }^2 C_1+{ }^2 C_2}{2!}+\frac{{ }^3 C_0+{ }^3 C_1+{ }^3 C_2+{ }^3 C_3}{3!}+\cdots
\end{aligned}
$$
Then $\frac{2 b}{a^2}$ is equal to
