Question Number 214511 by issac last updated on 11/Dec/24
$$\mathrm{Let}'\mathrm{s}\:{R}\left({z}\right)\:\mathrm{define}\:\mathrm{as}\: \\ $$$${R}\left({z}\right)=\frac{\pi\:\int_{\mathrm{0}} ^{\:{z}} \:{f}^{\mathrm{2}} \left({t}\right)\mathrm{d}{t}}{\mathrm{2}\pi\:\int_{\mathrm{0}} ^{\:{z}} \:{f}\left({t}\right)\sqrt{\mathrm{1}+\left({f}^{\left(\mathrm{1}\right)} \left({t}\right)\right)^{\mathrm{2}} }\mathrm{d}{t}} \\ $$$$\mathrm{and}\:\mathrm{both}\:\mathrm{integral} \\ $$$$\int_{\mathrm{0}} ^{\:\infty} \:{f}^{\:\mathrm{2}} \left({t}\right)\mathrm{d}{t}\:,\:\int_{\mathrm{0}} ^{\:\infty} \:{f}\left({t}\right)\sqrt{\mathrm{1}+\left({f}^{\left(\mathrm{1}\right)} \left({t}\right)\right)^{\mathrm{2}} }\mathrm{d}{t}\:=\infty \\ $$$$\underset{{z}\rightarrow\infty} {\mathrm{lim}}\:\frac{\pi\:\int_{\mathrm{0}} ^{\:{z}} {f}^{\:\mathrm{2}} \left({t}\right)\mathrm{d}{t}}{\mathrm{2}\pi\:\int_{\mathrm{0}} ^{\:{z}} \:{f}\left({t}\right)\sqrt{\mathrm{1}+\left({f}^{\left(\mathrm{1}\right)} \left({t}\right)\right)^{\mathrm{2}} }\mathrm{d}{t}} \\ $$$$=\underset{{z}\rightarrow\infty} {\mathrm{lim}}\:\frac{\pi\:{f}\left({z}\right)}{\mathrm{2}\pi\:\sqrt{\mathrm{1}+\left({f}^{\left(\mathrm{1}\right)} \left({z}\right)\right)^{\mathrm{2}} }}\:..?? \\ $$