If a function \( f(x) \) is given as:
\[
f(x) =
\begin{cases}
\frac{\sqrt{1+ax^2+bx^3}-\sqrt{1-ax^2-bx^3}}{x^2}, & x<0 \\
5, & x = 0 \\
\frac{\tan3x-\sin3x}{bx^3}, & x>0
\end{cases}
\]
and is continuous at \( x = 0 \), then the geometric mean of \( a \) and \( b \) is:
