设$ f(x)= \begin{cases} \dfrac { \arctan 3x+e^{x}-1}{x},&x < 0 \\ 1,&x=0 \\ \dfrac {1-(1-x^{2})^{b}}{x \arcsin 2x},&x>0 \end{cases} $，在$x=0$处连续,则$a=\underline{\quad\quad\quad}$,$b=\underline{\quad\quad\quad}$.
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