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\mathbf{D}\left(\frac{2}{3}(3 x+8)^{1 / 2}\right)=\frac{2}{3} \frac{1}{2} \cdot(3 x+8)^{-1 / 2} \mathbf{D}(3 x+8)=\frac{2}{3} \frac{3}{2} \cdot(3 x+8)^{-1 / 2}=(3 x+8)^{-1 / 2}