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\frac{\mathrm{d}}{\mathrm{dx}} \mathrm{A}(x)=\mathrm{f}(x), \text { then } \frac{\mathrm{d}}{\mathrm{dx}} \mathrm{A}\left(x^{2}\right)=\mathrm{f}\left(x^{2}\right) \cdot 2 x