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\frac{\mathrm{d} A}{d t}=x \frac{d y}{d t}+y \frac{d x}{d t}=(12 \, \mathrm{ft})(-2 \, \mathrm{ft} / \mathrm{sec})+(8 \mathrm{ft})(3 \, \mathrm{ft} / \mathrm{sec})=0 \, \mathrm{ft}^{2} / \mathrm{sec}