设$ \lim \limits _{x \rightarrow \infty } \left ( \dfrac {x+c}{x-c} \right )^{ \dfrac {2x^{2}+3x+2}{x+1}}= \lim \limits _{x \rightarrow 0}( \cos x+x^{2})^{ \dfrac {1}{ \sin ^{2}x}}$, 则$c=\underline{\quad\quad\quad}$.
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