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\frac{\mathrm{A}(2.1)-\mathrm{A}(2)}{0.1} \approx \mathrm{f}(2) \text { or } \mathrm{f}(2.1) \text { so } \frac{\mathrm{A}(2.1)-\mathrm{A}(2)}{0.1} \approx 1