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\mathbf{D}\left(\frac{2}{5} \sqrt{5 x+3}\right)=\frac{1}{5} \cdot \mathbf{D}(2 \sqrt{5 x+3})=\frac{1}{5} \cdot \mathbf{5} \cdot(5 x+3)^{-1 / 2}=\frac{1}{\sqrt{5 x+3}}