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\frac{\mathrm{d} \mathrm{C}}{\mathrm{dt}}=\frac{\mathrm{b} \frac{\mathrm{d} \mathrm{b}}{\mathrm{dt}}+\mathrm{h} \frac{\mathrm{d} \mathrm{h}}{\mathrm{dt}}}{\sqrt{\mathrm{b}^{2}+\mathrm{h}^{2}}}=\frac{15(3)+13(-3)}{\sqrt{15^{2}+13^{2}}} > 0