The function $f(x)= \frac{[ln(1+ax)-ln(1-b x)]}{x}$ is not defined at $x=0$. What value may be assigned to $f$ at $x=0$, so that it is continuous?

A function f(x) is defined as  $$f(x)=\begin{cases}{\frac{1-\cos 4x}{{x}^2}} & {;x{\lt}0} \\ {a} & {;x=0} \\ {\frac{\sqrt[]{x}}{\sqrt[]{(16+\sqrt[]{x})-4}}} & {;x{\gt}0}\end{cases}$$ if the function f(x) is continuous at x = 0, then the value of a is: