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设参数方程 $\left\{\begin{array}{l}x=\ln \left(1+t^2\right) \\ y=t-\arctan t\end{array}\right.$ , 求 $\frac{\mathrm{d} y}{\mathrm{~d} x}, \frac{\mathrm{d}^2 y}{\mathrm{~d} x^2}$.
                        
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