$\lim\limits_{x \to 0}\dfrac{\tan x - x}{x^{2}(e^{x}-1)}$.

设$f(x)=\begin{cases}a + x^{2},&x\leq0\\\dfrac{1}{x}\sin\dfrac{1}{x},&x > 0\end{cases}$在$x = 0$处连续，求$a$的值.

求不定积分$\int x^{2}\cos xdx$.