Explanation:
(D) : Given,
\(\mathrm{f}(\mathrm{x})=\frac{\mathrm{x}}{8}+\frac{2}{\mathrm{x}}, \mathrm{x} \in[1,6]\)
On differenting w.r.t \(\mathrm{x}\) we get
\(\mathrm{f}^{\prime}(\mathrm{x})=\frac{1}{8}-\frac{2}{\mathrm{x}^{2}}\)
For maxima or minima
\(\mathrm{f}^{\prime}(\mathrm{x})=0\)
\(\frac{1}{8}-\frac{2}{x^{2}}=0\)
\(\frac{2}{x^{2}}=\frac{1}{8}\)
\(x^{2}=16\)
\(x= \pm 4\)
\(x=4, x \in[1,6]\)
\(\mathrm{f}^{\prime}(4)=\frac{1}{8}-\frac{2}{16}\)
\(\frac{1}{8}-\frac{1}{8}\)
\(\mathrm{f}(4)=0\)
Also, in [1, 4], \(\mathrm{f}^{\prime}(\mathrm{x})\lt 0 \mathrm{f}(\mathrm{x})\) is decreasing in [4, 6], \(\mathrm{f}^{\prime}(\mathrm{x})\)
\(>0 \mathrm{f}(\mathrm{x})\) is increasing
Now, \(\quad \mathrm{f}(1)=\frac{1}{8}+\frac{2}{1}=\frac{17}{8}\)
\(f(6)=\frac{6}{8}+\frac{2}{6}=\frac{13}{12}\)
Hence, maximum value of \(f^{\prime}(x)\)
\(f(x)=\frac{17}{8} \quad \in[1,6]\)