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\quad \frac{\mathrm{dH}}{\mathrm{dt}}=\frac{\mathrm{dV} / \mathrm{dt}}{\pi \mathrm{R}^{2}}=\frac{\mathrm{K} \pi \mathrm{R}^{2}}{\pi \mathrm{R}^{2}}=\mathrm{K}