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Let \(\mathrm{f}:(0, \infty) \rightarrow \mathrm{R}\) be a twice differentiable function. If for some \(a \neq 0, \int_{0}^{1} f(\lambda x) d \lambda=\operatorname{af}(x),\) \(f(1)=1\) and \(f(16)=\frac{1}{8},\) then \(16-f^{\prime}\left(\frac{1}{16}\right)\) is equal to _______.

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Answer is: 112   

\(\int_{0}^{1} f(\lambda x) d \lambda=\operatorname{af}(x)\)

\(\lambda \mathrm{x}=\mathrm{t}\)

\(\mathrm{d} \lambda=\frac{1}{\mathrm{x}} \mathrm{dt}\)

\(\frac{1}{x} \int_{0}^{x} f(t) d t=a f(x)\)

\(\int_{0}^{x} f(t) d t=\operatorname{axf}(x)\)

\(f(x)=a\left(x f^{\prime}(x)+f(x)\right)\)

\((1-a) f(x)=a \cdot x f^{\prime}(x)\)

\(\frac{f^{\prime}(x)}{f(x)}=\frac{(1-a)}{a} \frac{1}{x}\)

\(\operatorname{\ell nf}(\mathrm{x})=\frac{1-\mathrm{a}}{\mathrm{a}} \operatorname{\ell n} \mathrm{x}+\mathrm{c}\)

\(\mathrm{x}=1, \mathrm{f}(1)=1 \Rightarrow \mathrm{c}=0\)

\(\mathrm{x}=16, \mathrm{f}(16)=\frac{1}{8}\)

\(\frac{1}{8}=(16)^{\frac{1-a}{a}} \Rightarrow-3=\frac{4-4 a}{a} \Rightarrow a=4\)

\(f(x)=x^{-\frac{3}{4}}\)

\(f^{\prime}(x)=-\frac{3}{4} x^{-\frac{7}{4}}\)

\(\therefore 16-\mathrm{f}^{\prime}\left(\frac{1}{16}\right)\)

\(=16-\left(-\frac{3}{4}\left(2^{-4}\right)^{-7 / 4}\right)\)

= 16 + 96 = 112

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