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Question Number 42603 by maxmathsup by imad last updated on 28/Aug/18

let f(x) =∫_0 ^1 ln(1+ixt)dt  calculate f^, (x)    (x from R).

$${let}\:{f}\left({x}\right)\:=\int_{\mathrm{0}} ^{\mathrm{1}} {ln}\left(\mathrm{1}+{ixt}\right){dt}\:\:{calculate}\:{f}^{,} \left({x}\right)\:\:\:\:\left({x}\:{from}\:{R}\right). \\ $$

Commented by prof Abdo imad last updated on 29/Aug/18

we have 1+ixt =(√(1+x^2 t^2 ))((1/(√(1+x^2 t^2 ))) +i((xt)/(√(1+x^2 t^2 ))))  =r e^(iθ)  ⇒r=(√(1+x^2 t^2 )) and cosθ =(1/(√(1+x^2 t^2 )))  sinθ =((xt)/(√(1+x^2 t^2 ))) ⇒tanθ =xt ⇒θ=arctan(xt)⇒  ln(1+xt) =ln(r)+iθ =(1/2)ln(1+x^2 t^2 )+i arctan(xt)  ⇒f(x)=(1/2)∫_0 ^1  ln(1+x^2 t^2 )dt +i ∫_0 ^1  arctan(xt)dt⇒  f^′ (x)=(1/2) ∫_0 ^1 ((2xt^2 )/(1+x^2 t^2 ))dt  +i ∫_0 ^1   (t/(1+x^2 t^2 ))dt .  = ∫_0 ^1    ((xt^2 )/(1+x^2 t^2 )) dt  +i  ∫_0 ^1   (t/(1+x^2 t^2 ))dt but for x≠0  ∫_0 ^1   ((xt^2 )/(1+x^2 t^2 )) dt =(1/x) ∫_0 ^1  ((x^2 t^2  +1−1)/(1+x^2 t^2 ))dt  =(1/x) −(1/x) ∫_0 ^1    (dt/(1+x^2 t^2 )) =_(xt =u)  (1/x) −(1/x)∫_0 ^x    (1/(1+u^2 ))(du/x)  =(1/x) −(1/x^2 ) arctan(x)  also  ∫_0 ^1     (t/(1+x^2 t^2 )) dt =_(xt =u)   ∫_0 ^x      (u/(x(1+u^2 ))) (du/x)  =(1/x^2 ) ∫_0 ^x   (u/(1+u^2 ))du =(1/(2x^2 ))ln∣1+x^2 ∣ ⇒  f^′ (x)=(1/x) −((arctanx)/x^2 )  +i{(1/(2x^2 ))ln(1+x^2 } .

$${we}\:{have}\:\mathrm{1}+{ixt}\:=\sqrt{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }\left(\frac{\mathrm{1}}{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }}\:+{i}\frac{{xt}}{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }}\right) \\ $$$$={r}\:{e}^{{i}\theta} \:\Rightarrow{r}=\sqrt{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }\:{and}\:{cos}\theta\:=\frac{\mathrm{1}}{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }} \\ $$$${sin}\theta\:=\frac{{xt}}{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }}\:\Rightarrow{tan}\theta\:={xt}\:\Rightarrow\theta={arctan}\left({xt}\right)\Rightarrow \\ $$$${ln}\left(\mathrm{1}+{xt}\right)\:={ln}\left({r}\right)+{i}\theta\:=\frac{\mathrm{1}}{\mathrm{2}}{ln}\left(\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} \right)+{i}\:{arctan}\left({xt}\right) \\ $$$$\Rightarrow{f}\left({x}\right)=\frac{\mathrm{1}}{\mathrm{2}}\int_{\mathrm{0}} ^{\mathrm{1}} \:{ln}\left(\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} \right){dt}\:+{i}\:\int_{\mathrm{0}} ^{\mathrm{1}} \:{arctan}\left({xt}\right){dt}\Rightarrow \\ $$$${f}^{'} \left({x}\right)=\frac{\mathrm{1}}{\mathrm{2}}\:\int_{\mathrm{0}} ^{\mathrm{1}} \frac{\mathrm{2}{xt}^{\mathrm{2}} }{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }{dt}\:\:+{i}\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{{t}}{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }{dt}\:. \\ $$$$=\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\:\frac{{xt}^{\mathrm{2}} }{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }\:{dt}\:\:+{i}\:\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{{t}}{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }{dt}\:{but}\:{for}\:{x}\neq\mathrm{0} \\ $$$$\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{{xt}^{\mathrm{2}} }{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }\:{dt}\:=\frac{\mathrm{1}}{{x}}\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\frac{{x}^{\mathrm{2}} {t}^{\mathrm{2}} \:+\mathrm{1}−\mathrm{1}}{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }{dt} \\ $$$$=\frac{\mathrm{1}}{{x}}\:−\frac{\mathrm{1}}{{x}}\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\:\frac{{dt}}{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }\:=_{{xt}\:={u}} \:\frac{\mathrm{1}}{{x}}\:−\frac{\mathrm{1}}{{x}}\int_{\mathrm{0}} ^{{x}} \:\:\:\frac{\mathrm{1}}{\mathrm{1}+{u}^{\mathrm{2}} }\frac{{du}}{{x}} \\ $$$$=\frac{\mathrm{1}}{{x}}\:−\frac{\mathrm{1}}{{x}^{\mathrm{2}} }\:{arctan}\left({x}\right)\:\:{also} \\ $$$$\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\:\:\frac{{t}}{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }\:{dt}\:=_{{xt}\:={u}} \:\:\int_{\mathrm{0}} ^{{x}} \:\:\:\:\:\frac{{u}}{{x}\left(\mathrm{1}+{u}^{\mathrm{2}} \right)}\:\frac{{du}}{{x}} \\ $$$$=\frac{\mathrm{1}}{{x}^{\mathrm{2}} }\:\int_{\mathrm{0}} ^{{x}} \:\:\frac{{u}}{\mathrm{1}+{u}^{\mathrm{2}} }{du}\:=\frac{\mathrm{1}}{\mathrm{2}{x}^{\mathrm{2}} }{ln}\mid\mathrm{1}+{x}^{\mathrm{2}} \mid\:\Rightarrow \\ $$$${f}^{'} \left({x}\right)=\frac{\mathrm{1}}{{x}}\:−\frac{{arctanx}}{{x}^{\mathrm{2}} }\:\:+{i}\left\{\frac{\mathrm{1}}{\mathrm{2}{x}^{\mathrm{2}} }{ln}\left(\mathrm{1}+{x}^{\mathrm{2}} \right\}\:.\right. \\ $$$$ \\ $$

Commented by prof Abdo imad last updated on 29/Aug/18

another method by complex derovation  f^′ (x) =∫_0 ^1   (∂/∂x)(ln(1+ixt))dt  = ∫_0 ^1    ((it)/(1+ixt))dt =∫_0 ^1   ((it(1−ixt))/(1+x^2 t^2 ))dt  = ∫_0 ^1    ((xt^2 )/(1+x^2 t^2 ))dt +i ∫_0 ^1   ((tdt)/(1+x^2 t^2 )) =....

$${another}\:{method}\:{by}\:{complex}\:{derovation} \\ $$$${f}^{'} \left({x}\right)\:=\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{\partial}{\partial{x}}\left({ln}\left(\mathrm{1}+{ixt}\right)\right){dt} \\ $$$$=\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\:\frac{{it}}{\mathrm{1}+{ixt}}{dt}\:=\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{{it}\left(\mathrm{1}−{ixt}\right)}{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }{dt} \\ $$$$=\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\:\frac{{xt}^{\mathrm{2}} }{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }{dt}\:+{i}\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{{tdt}}{\mathrm{1}+{x}^{\mathrm{2}} {t}^{\mathrm{2}} }\:=.... \\ $$

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