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求极限 $\lim _{x \rightarrow 0} \frac{\int_0^x\left[\int_0^{u^2} \sin t \cdot \arctan (1+t) \mathrm{d} t\right] \mathrm{d} u}{x^3(\sqrt[3]{1+x}-1)^2}$.
                        
不再提醒