Question and Answers Forum

All Questions      Topic List

Integration Questions

Previous in All Question      Next in All Question      

Previous in Integration      Next in Integration      

Question Number 136999 by mnjuly1970 last updated on 28/Mar/21

$$$$$$......advanced......calculus....$$$$\mathit{\varphi }={\int }_0^{\mathrm{\infty }}ln\left(x\right).sin\left(x\right).e^{-x}dx=?$$$$$$

Answered by mnjuly1970 last updated on 28/Mar/21

$$\mathit{\varphi }=-Im{\int }_0^{\mathrm{\infty }}ln\left(x\right)e^{-ix-x}dx$$$$=-Im{\int }_0^{\mathrm{\infty }}ln\left(x\right)e^{-x(1+i)}dx$$$$\stackrel{x(1+i)=y}{=}-\frac{1}{2}Im(1-i){\int }_0^{\mathrm{\infty }}(ln\left(y\right)-ln\left(\sqrt{2}{e}^{\frac{i\pi }{4}}\right))e^{-y}dy$$$$=\frac{-\sqrt{2}}{2}Im(\frac{\sqrt{2}}{2}-\frac{i\sqrt{2}}{2})\{{\int }_0^{\mathrm{\infty }}ln\left(y\right)e^{-y}dy+(ln\left(\sqrt{2}\right)+\frac{i\pi }{4}){\left[e^{-y}\right]}_0^{\mathrm{\infty }}\}$$$$=\frac{-\sqrt{2}}{2}Im\{(\frac{\sqrt{2}}{2}-\frac{i\sqrt{2}}{2})(-\gamma -ln\left(\sqrt{2}\right)-\frac{i\pi }{4})$$$$=\frac{-\sqrt{2}}{2}(\frac{\sqrt{2}}{2}.\left(\frac{-\pi }{4}\right)+\left(\frac{\gamma +ln\left(\sqrt{2}\right)}{2}\right)\sqrt{2})$$$$=\frac{\pi }{8}-\frac{\gamma }{2}-\frac{1}{4}ln\left(2\right).....✓$$$$......\mathit{\varphi }=\frac{\pi }{8}-\frac{1}{4}ln\left(2\right)-\frac{\gamma }{2}$$

Answered by Dwaipayan Shikari last updated on 28/Mar/21

$$\chi \left(\alpha \right)={\int }_0^{\mathrm{\infty }}{x}^{\alpha }{e}^{-x}sin\left(x\right)dx$$$${\chi }^{\prime }\left(0\right)={\int }_0^{\mathrm{\infty }}log\left(x\right)e^{-x}sin\left(x\right)dx$$$$\Rightarrow \chi \left(\alpha \right)=\frac{1}{2i}{\int }_0^{\mathrm{\infty }}{x}^{\alpha }{e}^{-(1-i)x}-x^{\alpha }{e}^{-(1+i)x}dx$$$$\Rightarrow \chi \left(\alpha \right)=\frac{1}{2i}.\frac{\mathrm{\Gamma }(\alpha +1)}{{(1-i)}^{\alpha +1}}-\frac{\mathrm{\Gamma }(\alpha +1)}{2i{(1+i)}^{\alpha +1}}\Rightarrow \chi \left(\alpha \right)=\frac{1}{2^{\frac{\alpha +3}{2}}i}(e^{\frac{\pi }{4}(\alpha +1)i}-e^{-\frac{\pi }{4}(\alpha +1)i})$$$$\Rightarrow \chi \left(\alpha \right)=\frac{sin\left(\frac{\pi }{4}(\alpha +1)\right)\mathrm{\Gamma }(\alpha +1)}{2^{\frac{\alpha +1}{2}}}$$$$\Rightarrow {\chi }^{\prime }\left(\alpha \right)=\frac{{\left(\sqrt{2}\right)}^{\alpha +1}\frac{\pi }{4}cos\left(\frac{\pi }{4}(\alpha +1)\right)\mathrm{\Gamma }(\alpha +1)+{\mathrm{\Gamma }}^{\prime }(\alpha +1){\left(\sqrt{2}\right)}^{\alpha +1}sin\left(\frac{\pi }{4}(\alpha +1)\right)-2^{\frac{a-1}{2}}log\left(2\right)sin\left(\frac{\pi }{4}(a+1)\right)\mathrm{\Gamma }(\alpha +1)}{2^{\alpha +1}}$$$${\chi }^{\prime }\left(0\right)=\frac{\frac{\pi }{4}-\gamma +\frac{log\left(2\right)}{2}}{2}=\frac{\pi }{8}-\frac{\gamma }{2}-\frac{log\left(2\right)}{4}$$$$$$

Commented by mnjuly1970 last updated on 28/Mar/21

$$thankyousomuch..$$

Terms of Service

Privacy Policy

Contact: info@tinkutara.com