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\frac{\mathrm{d} \mathrm{A}}{\mathrm{dt}}=\frac{1}{2}\left\{\mathrm{~b} \frac{\mathrm{d} \mathrm{h}}{\mathrm{dt}}+\mathrm{h} \frac{\mathrm{d} \mathrm{b}}{\mathrm{dt}}\right\}=\frac{1}{2}\{(15 \, \mathrm{in})(-3 \, \mathrm{in} / \mathrm{hr})+(13 \, \mathrm{in})(3 \, \mathrm{in} / \mathrm{hr})\} < 0