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 42810 by maxmathsup by imad last updated on 02/Sep/18

$$find{\int }_0^{\mathrm{\infty }}\frac{x^5}{1+x^7}dx.$$

Commented by prof Abdo imad last updated on 03/Sep/18

$$changementx=t^{\frac{1}{7}}give$$$$I={\int }_0^{\mathrm{\infty }}\frac{t^{\frac{5}{7}}}{1+t}\frac{1}{7}{t}^{\frac{1}{7}-1}dt$$$$=\frac{1}{7}{\int }_0^{\mathrm{\infty }}\frac{t^{\frac{6}{7}-1}}{1+t}dt=\frac{1}{7}\frac{\pi }{sin\left(\frac{6\pi }{7}\right)}=\frac{\pi }{7sin\left(\frac{\pi }{7}\right)}.$$

Answered by tanmay.chaudhury50@gmail.com last updated on 03/Sep/18

$$x^7={tan}^2\alpha x={\left(tan\alpha \right)}^{\frac{2}{7}}$$$$dx=\frac{2}{7}{tan}^{\frac{-5}{7}}\alpha {sec}^2\alpha d\alpha$$$${\int }_0^{\frac{\mathrm{\Pi }}{2}}\frac{{\left(tan\alpha \right)}^{\frac{10}{7}}}{{sec}^2\alpha }\times \frac{2}{7}{\left(tan\alpha \right)}^{\frac{-5}{7}}\times {sec}^2\alpha d\alpha$$$$\frac{2}{7}{\int }_0^{\frac{\mathrm{\Pi }}{2}}{\left(tan\alpha \right)}^{\frac{5}{7}}\alpha d\alpha$$$$\frac{2}{7}{\int }_0^{\frac{\mathrm{\Pi }}{2}}{\left(sin\alpha \right)}^{\frac{5}{7}}{\left(cos\alpha \right)}^{\frac{-5}{7}}d\alpha$$$$usingformula$$$${\int }_0^{\frac{\mathrm{\Pi }}{2}}{sin}^{2p-1}\alpha {cos}^{2q-1}\alpha d\alpha =\frac{\lceil p\times \lceil q}{2\lceil (p+q)}$$$$here2p-1=\frac{5}{7}2p=\frac{12}{7}p=\frac{6}{7}$$$$2q-1=-\frac{5}{7}2q=\frac{2}{7}q=\frac{1}{7}$$$$=\frac{2}{7}{\int }_0^{\frac{\mathrm{\Pi }}{2}}{\left(sin{\alpha }^{}\right)}^{2\times \frac{6}{7}-1}{\left(cos\alpha \right)}^{\frac{2\times 1}{7}-1}d\alpha$$$$\frac{2}{7}\times \frac{\lceil \left(\frac{6}{7}\right)\times \lceil \left(\frac{1}{7}\right)}{2\lceil (\frac{6}{7}+\frac{1}{7})}=\frac{1}{7}\times \lceil \left(\frac{1}{7}\right)\times \lceil (1-\frac{1}{7})=\frac{1}{7}\times \frac{\mathrm{\Pi }}{sin\left(\frac{\mathrm{\Pi }}{7}\right)}$$$$$$$$\lceil \left(p\right)\times \lceil (1-p)=\frac{\mathrm{\Pi }}{sin\left(p\mathrm{\Pi }\right)}$$

Terms of Service

Privacy Policy

Contact: info@tinkutara.com