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 132729 by mnjuly1970 last updated on 16/Feb/21

              ....advanced   calculus...      evaluate :      𝛗=∫_0 ^( ∞) xe^(−2x) ln(x)dx=???

$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:....{advanced}\:\:\:{calculus}... \\ $$$$\:\:\:\:{evaluate}\:: \\ $$$$\:\:\:\:\boldsymbol{\phi}=\int_{\mathrm{0}} ^{\:\infty} {xe}^{−\mathrm{2}{x}} {ln}\left({x}\right){dx}=??? \\ $$$$ \\ $$

Answered by Olaf last updated on 16/Feb/21

φ = ∫_0 ^∞ xe^(−2x) lnxdx  φ = ∫_0 ^∞ (xlnx−x)e^(−2x) dx+∫_0 ^∞ xe^(−2x) dx  φ = [−(e^(−2x) /2)(xlnx−x)]_0 ^∞ +(1/2)∫_0 ^∞ lnxe^(−2x) dx  +[−((1/2)x+(1/4))e^(−2x) ]_0 ^∞   φ = (1/2)∫_0 ^∞ lnxe^(−2x) dx+(1/4)  φ = (1/2)(−(γ/2)−((ln2)/2))+(1/4)  φ = (1/4)(1−γ−ln2)

$$\phi\:=\:\int_{\mathrm{0}} ^{\infty} {xe}^{−\mathrm{2}{x}} \mathrm{ln}{xdx} \\ $$$$\phi\:=\:\int_{\mathrm{0}} ^{\infty} \left({x}\mathrm{ln}{x}−{x}\right){e}^{−\mathrm{2}{x}} {dx}+\int_{\mathrm{0}} ^{\infty} {xe}^{−\mathrm{2}{x}} {dx} \\ $$$$\phi\:=\:\left[−\frac{{e}^{−\mathrm{2}{x}} }{\mathrm{2}}\left({x}\mathrm{ln}{x}−{x}\right)\right]_{\mathrm{0}} ^{\infty} +\frac{\mathrm{1}}{\mathrm{2}}\int_{\mathrm{0}} ^{\infty} \mathrm{ln}{xe}^{−\mathrm{2}{x}} {dx} \\ $$$$+\left[−\left(\frac{\mathrm{1}}{\mathrm{2}}{x}+\frac{\mathrm{1}}{\mathrm{4}}\right){e}^{−\mathrm{2}{x}} \right]_{\mathrm{0}} ^{\infty} \\ $$$$\phi\:=\:\frac{\mathrm{1}}{\mathrm{2}}\int_{\mathrm{0}} ^{\infty} \mathrm{ln}{xe}^{−\mathrm{2}{x}} {dx}+\frac{\mathrm{1}}{\mathrm{4}} \\ $$$$\phi\:=\:\frac{\mathrm{1}}{\mathrm{2}}\left(−\frac{\gamma}{\mathrm{2}}−\frac{\mathrm{ln2}}{\mathrm{2}}\right)+\frac{\mathrm{1}}{\mathrm{4}} \\ $$$$\phi\:=\:\frac{\mathrm{1}}{\mathrm{4}}\left(\mathrm{1}−\gamma−\mathrm{ln2}\right) \\ $$

Commented by mnjuly1970 last updated on 16/Feb/21

tayeballah mr olaf...

$${tayeballah}\:{mr}\:{olaf}... \\ $$

Answered by Dwaipayan Shikari last updated on 16/Feb/21

I(a)=∫_0 ^∞ x^(a−1) e^(−2x) dx=(1/2^a )∫_0 ^∞ u^(a−1) e^(−u) du=((Γ(a))/2^a )  I′(a)=((Γ′(a))/2^a )−((Γ(a))/2^a )log(2)  I′(2)=∫_0 ^∞ xe^(−2x) log(x)dx=((Γ(2)ψ(2))/4)−((Γ(2))/4)log(2)=(1/4)(1−γ−log(2))

$${I}\left({a}\right)=\int_{\mathrm{0}} ^{\infty} {x}^{{a}−\mathrm{1}} {e}^{−\mathrm{2}{x}} {dx}=\frac{\mathrm{1}}{\mathrm{2}^{{a}} }\int_{\mathrm{0}} ^{\infty} {u}^{{a}−\mathrm{1}} {e}^{−{u}} {du}=\frac{\Gamma\left({a}\right)}{\mathrm{2}^{{a}} } \\ $$$${I}'\left({a}\right)=\frac{\Gamma'\left({a}\right)}{\mathrm{2}^{{a}} }−\frac{\Gamma\left({a}\right)}{\mathrm{2}^{{a}} }{log}\left(\mathrm{2}\right) \\ $$$${I}'\left(\mathrm{2}\right)=\int_{\mathrm{0}} ^{\infty} {xe}^{−\mathrm{2}{x}} {log}\left({x}\right){dx}=\frac{\Gamma\left(\mathrm{2}\right)\psi\left(\mathrm{2}\right)}{\mathrm{4}}−\frac{\Gamma\left(\mathrm{2}\right)}{\mathrm{4}}{log}\left(\mathrm{2}\right)=\frac{\mathrm{1}}{\mathrm{4}}\left(\mathrm{1}−\gamma−{log}\left(\mathrm{2}\right)\right) \\ $$

Answered by mnjuly1970 last updated on 16/Feb/21

Terms of Service

Privacy Policy

Contact: info@tinkutara.com