\( \text { If the function } f(x)=\left\{\begin{array}{cc} \frac{1}{x} \log _e\left(\frac{1+\frac{x}{a}}{1-\frac{x}{b}}\…

\(
\text { If the function } f(x)=\left\{\begin{array}{cc}
\frac{1}{x} \log _e\left(\frac{1+\frac{x}{a}}{1-\frac{x}{b}}\right) & , x<0 \\
k & , x=0 \\
\frac{\cos ^2 x-\sin ^2 x-1}{\sqrt{x^2+1}-1} & , x>0
\end{array}\right.
\)

\(
\text { is continuous at } x =0 \text {, then } \frac{1}{a}+\frac{1}{b}+\frac{4}{k} \text { is equal to: }
\)

[JEE Main 2021, 31 Aug (Shift 1)]