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Question Number 35241 by abdo mathsup 649 cc last updated on 17/May/18

calculate ∫_0 ^π     ((x dx)/(3 +cosx))  .

$${calculate}\:\int_{\mathrm{0}} ^{\pi} \:\:\:\:\frac{{x}\:{dx}}{\mathrm{3}\:+{cosx}}\:\:. \\ $$

Commented by abdo.msup.com last updated on 17/May/18

let put I=∫_0 ^π    ((xdx)/(3 +cosx)) .changement  tan((x/2))=t give  x =2arctan(t) and  I = ∫_0 ^∞      ((2arctan(t))/(3 +((1−t^2 )/(2+t^2 )))) ((2dt)/(1+t^2 ))  = 4∫_0 ^∞     ((arctan(t))/(3 +3t^2  +1−t^2 ))dt  =4 ∫_0 ^∞     ((arctan(t))/(4 +2t^2 ))dt=2∫_0 ^∞   ((arctan(t))/(2 +t^2 ))  =_(t=x(√2))    2∫_0 ^∞     ((arctan(x(√2)))/(2(1+x^2 )))(√2) dx  =(√2) ∫_0 ^∞      ((arctan(x(√2)))/(1+x^2 ))dx let introduce  the parametric function  f(t) =∫_0 ^∞     ((arctan(tx))/(1+x^2 ))dx  f^′ (t) = ∫_0 ^∞     (x/((1+t^2 x^2 )(1+x^2 )))dx  =_(tx =u)     ∫_0 ^∞      (u/(t(1+u^2 )(1+(u^2 /t^2 )))) (du/t)  = ∫_0 ^∞       (u/((1+u^2 )(t^2  +u^2 )))du  let  decompose g(u)= (u/((u^2  +1)(u^2  +t^2 )))  g(u) = ((au +b)/(u^2  +1))  +((cu +d)/(u^2  +t^2 ))  g(−u) =−g(u) ⇒b=d=0 so  g(u)= ((au)/(u^2 +1))  +((cu)/(u^2  +t^2 ))  lim_(u→+∞)  u g(u)=0 = a+c ⇒c=−a  g(u)= ((au)/(u^2 +1)) −((au)/(u^2  +t^2 ))  g(1)=(1/(2(1+t^2 ))) = (a/2) −(a/(t^2 +1)) ⇒   (1/2) =(1/2)(1+t^2 )a −a ⇒  1=(1+t^2 )a −2a =(t^2 −1)a ⇒  a= (1/(t^2 −1))    if  t^2 ≠1  so  g(u) = (1/(t^2 −1)) (u/(1+u^2 )) −(1/(t^2 −1)) (u/(t^2 +u^2 ))  f^′ (t) =∫_0 ^∞  g(u)du  =(1/(1−t^2 )){   ∫_0 ^∞   (u/(1+u^2 ))du −∫_0 ^∞    (u/(u^2  +t^2 ))du}  = (1/(2(1−t^2 )))[  ln(1+u^2 )−ln(u^2  +t^2 )]_0 ^(+∞)   = (1/(2(1−t^2 )))[ln(((u^2 +1)/(u^2  +t^2 )))]_0 ^(+∞)   = (1/(2(1−t^2 ))) {ln(t^2 )= ((ln(t))/(1−t^2 ))  f^′ (t) =  ((ln(t))/(1−t^2 ))  ⇒f(t)= ∫_0 ^t   ((lnx)/(1−x^2 ))dx +λ  λ=f(0)=0 ⇒ f(t)= ∫_0 ^t    ((ln(x))/(1−x^2 ))dx  I =(√2) f((√2))= (√2) ∫_0 ^(√2)     ((ln(x))/(1−x^2 ))dx...  be continued...

$${let}\:{put}\:{I}=\int_{\mathrm{0}} ^{\pi} \:\:\:\frac{{xdx}}{\mathrm{3}\:+{cosx}}\:.{changement} \\ $$$${tan}\left(\frac{{x}}{\mathrm{2}}\right)={t}\:{give}\:\:{x}\:=\mathrm{2}{arctan}\left({t}\right)\:{and} \\ $$$${I}\:=\:\int_{\mathrm{0}} ^{\infty} \:\:\:\:\:\frac{\mathrm{2}{arctan}\left({t}\right)}{\mathrm{3}\:+\frac{\mathrm{1}−{t}^{\mathrm{2}} }{\mathrm{2}+{t}^{\mathrm{2}} }}\:\frac{\mathrm{2}{dt}}{\mathrm{1}+{t}^{\mathrm{2}} } \\ $$$$=\:\mathrm{4}\int_{\mathrm{0}} ^{\infty} \:\:\:\:\frac{{arctan}\left({t}\right)}{\mathrm{3}\:+\mathrm{3}{t}^{\mathrm{2}} \:+\mathrm{1}−{t}^{\mathrm{2}} }{dt} \\ $$$$=\mathrm{4}\:\int_{\mathrm{0}} ^{\infty} \:\:\:\:\frac{{arctan}\left({t}\right)}{\mathrm{4}\:+\mathrm{2}{t}^{\mathrm{2}} }{dt}=\mathrm{2}\int_{\mathrm{0}} ^{\infty} \:\:\frac{{arctan}\left({t}\right)}{\mathrm{2}\:+{t}^{\mathrm{2}} } \\ $$$$=_{{t}={x}\sqrt{\mathrm{2}}} \:\:\:\mathrm{2}\int_{\mathrm{0}} ^{\infty} \:\:\:\:\frac{{arctan}\left({x}\sqrt{\mathrm{2}}\right)}{\mathrm{2}\left(\mathrm{1}+{x}^{\mathrm{2}} \right)}\sqrt{\mathrm{2}}\:{dx} \\ $$$$=\sqrt{\mathrm{2}}\:\int_{\mathrm{0}} ^{\infty} \:\:\:\:\:\frac{{arctan}\left({x}\sqrt{\mathrm{2}}\right)}{\mathrm{1}+{x}^{\mathrm{2}} }{dx}\:{let}\:{introduce} \\ $$$${the}\:{parametric}\:{function} \\ $$$${f}\left({t}\right)\:=\int_{\mathrm{0}} ^{\infty} \:\:\:\:\frac{{arctan}\left({tx}\right)}{\mathrm{1}+{x}^{\mathrm{2}} }{dx} \\ $$$${f}^{'} \left({t}\right)\:=\:\int_{\mathrm{0}} ^{\infty} \:\:\:\:\frac{{x}}{\left(\mathrm{1}+{t}^{\mathrm{2}} {x}^{\mathrm{2}} \right)\left(\mathrm{1}+{x}^{\mathrm{2}} \right)}{dx} \\ $$$$=_{{tx}\:={u}} \:\:\:\:\int_{\mathrm{0}} ^{\infty} \:\:\:\:\:\frac{{u}}{{t}\left(\mathrm{1}+{u}^{\mathrm{2}} \right)\left(\mathrm{1}+\frac{{u}^{\mathrm{2}} }{{t}^{\mathrm{2}} }\right)}\:\frac{{du}}{{t}} \\ $$$$=\:\int_{\mathrm{0}} ^{\infty} \:\:\:\:\:\:\frac{{u}}{\left(\mathrm{1}+{u}^{\mathrm{2}} \right)\left({t}^{\mathrm{2}} \:+{u}^{\mathrm{2}} \right)}{du}\:\:{let} \\ $$$${decompose}\:{g}\left({u}\right)=\:\frac{{u}}{\left({u}^{\mathrm{2}} \:+\mathrm{1}\right)\left({u}^{\mathrm{2}} \:+{t}^{\mathrm{2}} \right)} \\ $$$${g}\left({u}\right)\:=\:\frac{{au}\:+{b}}{{u}^{\mathrm{2}} \:+\mathrm{1}}\:\:+\frac{{cu}\:+{d}}{{u}^{\mathrm{2}} \:+{t}^{\mathrm{2}} } \\ $$$${g}\left(−{u}\right)\:=−{g}\left({u}\right)\:\Rightarrow{b}={d}=\mathrm{0}\:{so} \\ $$$${g}\left({u}\right)=\:\frac{{au}}{{u}^{\mathrm{2}} +\mathrm{1}}\:\:+\frac{{cu}}{{u}^{\mathrm{2}} \:+{t}^{\mathrm{2}} } \\ $$$${lim}_{{u}\rightarrow+\infty} \:{u}\:{g}\left({u}\right)=\mathrm{0}\:=\:{a}+{c}\:\Rightarrow{c}=−{a} \\ $$$${g}\left({u}\right)=\:\frac{{au}}{{u}^{\mathrm{2}} +\mathrm{1}}\:−\frac{{au}}{{u}^{\mathrm{2}} \:+{t}^{\mathrm{2}} } \\ $$$${g}\left(\mathrm{1}\right)=\frac{\mathrm{1}}{\mathrm{2}\left(\mathrm{1}+{t}^{\mathrm{2}} \right)}\:=\:\frac{{a}}{\mathrm{2}}\:−\frac{{a}}{{t}^{\mathrm{2}} +\mathrm{1}}\:\Rightarrow \\ $$$$\:\frac{\mathrm{1}}{\mathrm{2}}\:=\frac{\mathrm{1}}{\mathrm{2}}\left(\mathrm{1}+{t}^{\mathrm{2}} \right){a}\:−{a}\:\Rightarrow \\ $$$$\mathrm{1}=\left(\mathrm{1}+{t}^{\mathrm{2}} \right){a}\:−\mathrm{2}{a}\:=\left({t}^{\mathrm{2}} −\mathrm{1}\right){a}\:\Rightarrow \\ $$$${a}=\:\frac{\mathrm{1}}{{t}^{\mathrm{2}} −\mathrm{1}}\:\:\:\:{if}\:\:{t}^{\mathrm{2}} \neq\mathrm{1}\:\:{so} \\ $$$${g}\left({u}\right)\:=\:\frac{\mathrm{1}}{{t}^{\mathrm{2}} −\mathrm{1}}\:\frac{{u}}{\mathrm{1}+{u}^{\mathrm{2}} }\:−\frac{\mathrm{1}}{{t}^{\mathrm{2}} −\mathrm{1}}\:\frac{{u}}{{t}^{\mathrm{2}} +{u}^{\mathrm{2}} } \\ $$$${f}^{'} \left({t}\right)\:=\int_{\mathrm{0}} ^{\infty} \:{g}\left({u}\right){du} \\ $$$$=\frac{\mathrm{1}}{\mathrm{1}−{t}^{\mathrm{2}} }\left\{\:\:\:\int_{\mathrm{0}} ^{\infty} \:\:\frac{{u}}{\mathrm{1}+{u}^{\mathrm{2}} }{du}\:−\int_{\mathrm{0}} ^{\infty} \:\:\:\frac{{u}}{{u}^{\mathrm{2}} \:+{t}^{\mathrm{2}} }{du}\right\} \\ $$$$=\:\frac{\mathrm{1}}{\mathrm{2}\left(\mathrm{1}−{t}^{\mathrm{2}} \right)}\left[\:\:{ln}\left(\mathrm{1}+{u}^{\mathrm{2}} \right)−{ln}\left({u}^{\mathrm{2}} \:+{t}^{\mathrm{2}} \right)\right]_{\mathrm{0}} ^{+\infty} \\ $$$$=\:\frac{\mathrm{1}}{\mathrm{2}\left(\mathrm{1}−{t}^{\mathrm{2}} \right)}\left[{ln}\left(\frac{{u}^{\mathrm{2}} +\mathrm{1}}{{u}^{\mathrm{2}} \:+{t}^{\mathrm{2}} }\right)\right]_{\mathrm{0}} ^{+\infty} \\ $$$$=\:\frac{\mathrm{1}}{\mathrm{2}\left(\mathrm{1}−{t}^{\mathrm{2}} \right)}\:\left\{{ln}\left({t}^{\mathrm{2}} \right)=\:\frac{{ln}\left({t}\right)}{\mathrm{1}−{t}^{\mathrm{2}} }\right. \\ $$$${f}^{'} \left({t}\right)\:=\:\:\frac{{ln}\left({t}\right)}{\mathrm{1}−{t}^{\mathrm{2}} }\:\:\Rightarrow{f}\left({t}\right)=\:\int_{\mathrm{0}} ^{{t}} \:\:\frac{{lnx}}{\mathrm{1}−{x}^{\mathrm{2}} }{dx}\:+\lambda \\ $$$$\lambda={f}\left(\mathrm{0}\right)=\mathrm{0}\:\Rightarrow\:{f}\left({t}\right)=\:\int_{\mathrm{0}} ^{{t}} \:\:\:\frac{{ln}\left({x}\right)}{\mathrm{1}−{x}^{\mathrm{2}} }{dx} \\ $$$${I}\:=\sqrt{\mathrm{2}}\:{f}\left(\sqrt{\mathrm{2}}\right)=\:\sqrt{\mathrm{2}}\:\int_{\mathrm{0}} ^{\sqrt{\mathrm{2}}} \:\:\:\:\frac{{ln}\left({x}\right)}{\mathrm{1}−{x}^{\mathrm{2}} }{dx}... \\ $$$${be}\:{continued}... \\ $$

Commented by abdo.msup.com last updated on 17/May/18

error  change 1−t^2  by t^2 −1 so  f(t)= ∫_0 ^t    ((ln(x))/(x^2 −1))dx  I =(√2) ∫_0 ^(√2)   ((ln(x))/(x^2 −1))dx.

$${error}\:\:{change}\:\mathrm{1}−{t}^{\mathrm{2}} \:{by}\:{t}^{\mathrm{2}} −\mathrm{1}\:{so} \\ $$$${f}\left({t}\right)=\:\int_{\mathrm{0}} ^{{t}} \:\:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} −\mathrm{1}}{dx} \\ $$$${I}\:=\sqrt{\mathrm{2}}\:\int_{\mathrm{0}} ^{\sqrt{\mathrm{2}}} \:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} −\mathrm{1}}{dx}. \\ $$

Commented by abdo mathsup 649 cc last updated on 20/May/18

∫_0 ^(√2)    ((lnx)/(x^2 −1))dx = −∫_0 ^1   ((lnx)/(1−x^2 ))dx  +∫_1 ^(√2)  ((lnx)/(x^2  −1))dx but   ∫_0 ^1  ((ln(x))/(1−x^2 ))dx =∫_0 ^1  (Σ_(n=0) ^∞  x^(2n) )lnxdx  =Σ_(n=0) ^∞   ∫_0 ^1   x^(2n)  ln(x)dx   by parts  A_n   = ∫_0 ^1  x^(2n)  ln(x)dx =[(1/(2n+1))x^(2n+1) ln(x)]_0 ^1   − ∫_0 ^1   (1/(2n+1))x^(2n+1)  (1/x)dx = −(1/((2n+1)^2 )) ∫_0 ^1   x^(2n) dx  =−(1/((2n+1)^2 )) ⇒ ∫_0 ^1  ((ln(x))/(1−x^2 ))dx =−Σ_(n=0) ^∞   (1/((2n+1)^2 ))  we have (π^2 /6) =Σ_(n=1) ^∞    (1/n^2 ) =Σ_(n=1) ^∞   (1/((2n)^2 )) +Σ_(n=0) ^∞   (1/((2n+1)^2 ))  =(1/4) Σ_(n=1) ^∞  (1/n^2 ) +Σ_(n=0) ^∞    (1/((2n+1)^2 )) ⇒  Σ_(n=0) ^∞   (1/((2n+1)^2 )) = (3/4) Σ_(n=1) ^∞  (1/n^2 ) =(3/4) (π^2 /6) = (π^2 /8)

$$\int_{\mathrm{0}} ^{\sqrt{\mathrm{2}}} \:\:\:\frac{{lnx}}{{x}^{\mathrm{2}} −\mathrm{1}}{dx}\:=\:−\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{{lnx}}{\mathrm{1}−{x}^{\mathrm{2}} }{dx}\:\:+\int_{\mathrm{1}} ^{\sqrt{\mathrm{2}}} \:\frac{{lnx}}{{x}^{\mathrm{2}} \:−\mathrm{1}}{dx}\:{but} \\ $$$$\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\frac{{ln}\left({x}\right)}{\mathrm{1}−{x}^{\mathrm{2}} }{dx}\:=\int_{\mathrm{0}} ^{\mathrm{1}} \:\left(\sum_{{n}=\mathrm{0}} ^{\infty} \:{x}^{\mathrm{2}{n}} \right){lnxdx} \\ $$$$=\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:{x}^{\mathrm{2}{n}} \:{ln}\left({x}\right){dx}\:\:\:{by}\:{parts} \\ $$$${A}_{{n}} \:\:=\:\int_{\mathrm{0}} ^{\mathrm{1}} \:{x}^{\mathrm{2}{n}} \:{ln}\left({x}\right){dx}\:=\left[\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}{x}^{\mathrm{2}{n}+\mathrm{1}} {ln}\left({x}\right)\right]_{\mathrm{0}} ^{\mathrm{1}} \\ $$$$−\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}{x}^{\mathrm{2}{n}+\mathrm{1}} \:\frac{\mathrm{1}}{{x}}{dx}\:=\:−\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} }\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:{x}^{\mathrm{2}{n}} {dx} \\ $$$$=−\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} }\:\Rightarrow\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\frac{{ln}\left({x}\right)}{\mathrm{1}−{x}^{\mathrm{2}} }{dx}\:=−\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} } \\ $$$${we}\:{have}\:\frac{\pi^{\mathrm{2}} }{\mathrm{6}}\:=\sum_{{n}=\mathrm{1}} ^{\infty} \:\:\:\frac{\mathrm{1}}{{n}^{\mathrm{2}} }\:=\sum_{{n}=\mathrm{1}} ^{\infty} \:\:\frac{\mathrm{1}}{\left(\mathrm{2}{n}\right)^{\mathrm{2}} }\:+\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} } \\ $$$$=\frac{\mathrm{1}}{\mathrm{4}}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\frac{\mathrm{1}}{{n}^{\mathrm{2}} }\:+\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\:\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} }\:\Rightarrow \\ $$$$\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} }\:=\:\frac{\mathrm{3}}{\mathrm{4}}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\frac{\mathrm{1}}{{n}^{\mathrm{2}} }\:=\frac{\mathrm{3}}{\mathrm{4}}\:\frac{\pi^{\mathrm{2}} }{\mathrm{6}}\:=\:\frac{\pi^{\mathrm{2}} }{\mathrm{8}} \\ $$

Commented by abdo mathsup 649 cc last updated on 20/May/18

⇒ ∫_0 ^1   ((ln(x))/(x^2 −1))dx =−(π^2 /8)  changement x=(1/t) give  ∫_1 ^(√2)     ((ln(x))/(x^2 −1))dx =  ∫_1 ^(1/(√2))     ((−ln(t))/(( (1/t^2 ) −1))) (−(dt/t^2 ))  = −∫_(1/(√2)) ^1     ((ln(t))/(1−t^2 ))dt   =−∫_(1/(√2)) ^1  (Σ_(n=0) ^∞  t^(2n) )ln(t)dt  =−Σ_(n=0) ^∞   ∫_(1/(√2)) ^1   t^(2n)  ln(t)dt =−Σ_(n=0) ^∞  A_n    by parts  A_(n ) =  [ (1/(2n+1))t^(2n+1) ln(t)]_(1/(√2)) ^1  − ∫_(1/(√2)) ^1    (1/(2n+1))t^(2n) dt  =(1/(2n+1))((1/(√2)))^(2n+1) ln((√2)) −(1/((2n+1)^2 )) ⇒  Σ_(n=0) ^∞   A_n   = ln((√2))Σ_(n=0) ^∞  (1/(2n+1))((1/(√2)))^(2n+1)  −Σ_(n=0) ^∞  (1/((2n+1)^2 ))  ⇒ ∫_1 ^(√2)    ((ln(x))/(x^2 −1))dx = ln((√2))Σ_(n=0) ^∞  (1/((2n+1)^2 ))((1/(√2)))^(2n+1)     −(π^2 /8)

$$\Rightarrow\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} −\mathrm{1}}{dx}\:=−\frac{\pi^{\mathrm{2}} }{\mathrm{8}} \\ $$$${changement}\:{x}=\frac{\mathrm{1}}{{t}}\:{give} \\ $$$$\int_{\mathrm{1}} ^{\sqrt{\mathrm{2}}} \:\:\:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} −\mathrm{1}}{dx}\:=\:\:\int_{\mathrm{1}} ^{\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}} \:\:\:\:\frac{−{ln}\left({t}\right)}{\left(\:\frac{\mathrm{1}}{{t}^{\mathrm{2}} }\:−\mathrm{1}\right)}\:\left(−\frac{{dt}}{{t}^{\mathrm{2}} }\right) \\ $$$$=\:−\int_{\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}} ^{\mathrm{1}} \:\:\:\:\frac{{ln}\left({t}\right)}{\mathrm{1}−{t}^{\mathrm{2}} }{dt}\:\:\:=−\int_{\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}} ^{\mathrm{1}} \:\left(\sum_{{n}=\mathrm{0}} ^{\infty} \:{t}^{\mathrm{2}{n}} \right){ln}\left({t}\right){dt} \\ $$$$=−\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\int_{\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}} ^{\mathrm{1}} \:\:{t}^{\mathrm{2}{n}} \:{ln}\left({t}\right){dt}\:=−\sum_{{n}=\mathrm{0}} ^{\infty} \:{A}_{{n}} \:\:\:{by}\:{parts} \\ $$$${A}_{{n}\:} =\:\:\left[\:\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}{t}^{\mathrm{2}{n}+\mathrm{1}} {ln}\left({t}\right)\right]_{\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}} ^{\mathrm{1}} \:−\:\int_{\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}} ^{\mathrm{1}} \:\:\:\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}{t}^{\mathrm{2}{n}} {dt} \\ $$$$=\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}\left(\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)^{\mathrm{2}{n}+\mathrm{1}} {ln}\left(\sqrt{\mathrm{2}}\right)\:−\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} }\:\Rightarrow \\ $$$$\sum_{{n}=\mathrm{0}} ^{\infty} \:\:{A}_{{n}} \:\:=\:{ln}\left(\sqrt{\mathrm{2}}\right)\sum_{{n}=\mathrm{0}} ^{\infty} \:\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}\left(\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)^{\mathrm{2}{n}+\mathrm{1}} \:−\sum_{{n}=\mathrm{0}} ^{\infty} \:\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} } \\ $$$$\Rightarrow\:\int_{\mathrm{1}} ^{\sqrt{\mathrm{2}}} \:\:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} −\mathrm{1}}{dx}\:=\:{ln}\left(\sqrt{\mathrm{2}}\right)\sum_{{n}=\mathrm{0}} ^{\infty} \:\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} }\left(\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)^{\mathrm{2}{n}+\mathrm{1}} \:\: \\ $$$$−\frac{\pi^{\mathrm{2}} }{\mathrm{8}} \\ $$$$ \\ $$

Commented by prof Abdo imad last updated on 20/May/18

∫_1 ^(√2)     ((ln(x))/(x^2 −1))dx = ln((√2))Σ_(n=0) ^∞    (1/(2n+1))((1/(√2)))^(2n+1) −(π^2 /8)  let calculate the value of Σ_(n=0) ^∞   (1/(2n+1))((1/(√2)))^(2n+1)   let put S(x)= Σ_(n=0) ^∞   (1/(2n+1))x^(2n+1)   with ∣x∣<1  S^′ (x) = Σ_(n=0) ^∞    x^(2n)   = (1/(1−x^2 )) ⇒ S(x)= ∫ (dx/(1−x^2 )) +c  S(x)= (1/2)∫ {  (1/(1−x)) +(1/(1+x))}dx +c  =(1/2)ln∣((1+x)/(1−x))∣ +c  we have S(o)=0 ⇒c=0 ⇒  S(x) = (1/2)ln∣((1+x)/(1−x))∣ and  Σ_(n=0) ^∞   (1/(2n+1))((1/(√2)))^(2n+1)  = S((1/(√2)))   =(1/2)ln∣ ((1 +(1/(√2)))/(1−(1/(√2))))∣ = (1/2)ln((((√2) +1)/((√2) −1))) ⇒  ∫_1 ^(√2)      ((ln(x))/(x^2 −1))dx =((ln((√2)))/2)ln((((√2) +1)/((√2) −1))) −(π^2 /8)

$$\int_{\mathrm{1}} ^{\sqrt{\mathrm{2}}} \:\:\:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} −\mathrm{1}}{dx}\:=\:{ln}\left(\sqrt{\mathrm{2}}\right)\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\:\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}\left(\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)^{\mathrm{2}{n}+\mathrm{1}} −\frac{\pi^{\mathrm{2}} }{\mathrm{8}} \\ $$$${let}\:{calculate}\:{the}\:{value}\:{of}\:\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}\left(\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)^{\mathrm{2}{n}+\mathrm{1}} \\ $$$${let}\:{put}\:{S}\left({x}\right)=\:\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}{x}^{\mathrm{2}{n}+\mathrm{1}} \:\:{with}\:\mid{x}\mid<\mathrm{1} \\ $$$${S}^{'} \left({x}\right)\:=\:\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\:{x}^{\mathrm{2}{n}} \:\:=\:\frac{\mathrm{1}}{\mathrm{1}−{x}^{\mathrm{2}} }\:\Rightarrow\:{S}\left({x}\right)=\:\int\:\frac{{dx}}{\mathrm{1}−{x}^{\mathrm{2}} }\:+{c} \\ $$$${S}\left({x}\right)=\:\frac{\mathrm{1}}{\mathrm{2}}\int\:\left\{\:\:\frac{\mathrm{1}}{\mathrm{1}−{x}}\:+\frac{\mathrm{1}}{\mathrm{1}+{x}}\right\}{dx}\:+{c} \\ $$$$=\frac{\mathrm{1}}{\mathrm{2}}{ln}\mid\frac{\mathrm{1}+{x}}{\mathrm{1}−{x}}\mid\:+{c}\:\:{we}\:{have}\:{S}\left({o}\right)=\mathrm{0}\:\Rightarrow{c}=\mathrm{0}\:\Rightarrow \\ $$$${S}\left({x}\right)\:=\:\frac{\mathrm{1}}{\mathrm{2}}{ln}\mid\frac{\mathrm{1}+{x}}{\mathrm{1}−{x}}\mid\:{and} \\ $$$$\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\frac{\mathrm{1}}{\mathrm{2}{n}+\mathrm{1}}\left(\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)^{\mathrm{2}{n}+\mathrm{1}} \:=\:{S}\left(\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)\: \\ $$$$=\frac{\mathrm{1}}{\mathrm{2}}{ln}\mid\:\frac{\mathrm{1}\:+\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}}{\mathrm{1}−\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}}\mid\:=\:\frac{\mathrm{1}}{\mathrm{2}}{ln}\left(\frac{\sqrt{\mathrm{2}}\:+\mathrm{1}}{\sqrt{\mathrm{2}}\:−\mathrm{1}}\right)\:\Rightarrow \\ $$$$\int_{\mathrm{1}} ^{\sqrt{\mathrm{2}}} \:\:\:\:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} −\mathrm{1}}{dx}\:=\frac{{ln}\left(\sqrt{\mathrm{2}}\right)}{\mathrm{2}}{ln}\left(\frac{\sqrt{\mathrm{2}}\:+\mathrm{1}}{\sqrt{\mathrm{2}}\:−\mathrm{1}}\right)\:−\frac{\pi^{\mathrm{2}} }{\mathrm{8}} \\ $$

Commented by prof Abdo imad last updated on 20/May/18

so I =(√2)   ∫_0 ^(√2)    ((ln(x))/(x^2  −1))dx  =(√2){ ∫_0 ^1    ((ln(x))/(x^2 −1))dx + ∫_1 ^(√2)    ((ln(x))/(x^2  −1))dx}  =(√2_ ) { −(π^2 /8)  +((ln((√2)))/2)ln((((√2) +1)/((√2) −1))) −(π^2 /8)}  I =(√2){  ((ln((√2)))/2)ln((((√2) +1)/((√2) −1))) −(π^2 /4)}

$${so}\:{I}\:=\sqrt{\mathrm{2}}\:\:\:\int_{\mathrm{0}} ^{\sqrt{\mathrm{2}}} \:\:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} \:−\mathrm{1}}{dx} \\ $$$$=\sqrt{\mathrm{2}}\left\{\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} −\mathrm{1}}{dx}\:+\:\int_{\mathrm{1}} ^{\sqrt{\mathrm{2}}} \:\:\:\frac{{ln}\left({x}\right)}{{x}^{\mathrm{2}} \:−\mathrm{1}}{dx}\right\} \\ $$$$=\sqrt{\mathrm{2}_{} }\:\left\{\:−\frac{\pi^{\mathrm{2}} }{\mathrm{8}}\:\:+\frac{{ln}\left(\sqrt{\mathrm{2}}\right)}{\mathrm{2}}{ln}\left(\frac{\sqrt{\mathrm{2}}\:+\mathrm{1}}{\sqrt{\mathrm{2}}\:−\mathrm{1}}\right)\:−\frac{\pi^{\mathrm{2}} }{\mathrm{8}}\right\} \\ $$$${I}\:=\sqrt{\mathrm{2}}\left\{\:\:\frac{{ln}\left(\sqrt{\mathrm{2}}\right)}{\mathrm{2}}{ln}\left(\frac{\sqrt{\mathrm{2}}\:+\mathrm{1}}{\sqrt{\mathrm{2}}\:−\mathrm{1}}\right)\:−\frac{\pi^{\mathrm{2}} }{\mathrm{4}}\right\} \\ $$

Answered by math1967 last updated on 17/May/18

∫_(0 ) ^π ((xdx)/(3+2cos^2 (x/2)−1))=∫_(0 ) ^π ((xdx)/(2(1+cos^2 (x/2))))  now 2I=π∫_0 ^π (dx/(2(1+cos^2 (x/2))))  =(π/2)∫_0 ^π ((sec^2 (x/2)dx)/(sec^2 (x/2)+1))  =π∫_0 ^π (((1/2)sec^2 (x/2)dx)/(2+tan^2 (x/2)))  =π∫_0 ^π ((d(tan(x/2)))/(((√2))^2 +tan^2 (x/2)))=π[(1/(√2))tan^(−1) tan (x/(2(√2)))]_0 ^π   =π×[(1/(√2))×(π/(2(√2)))]=(π^2 /4)∴I=(π^2 /8)  plz. check

$$\underset{\mathrm{0}\:} {\overset{\pi} {\int}}\frac{{xdx}}{\mathrm{3}+\mathrm{2cos}\:^{\mathrm{2}} \frac{{x}}{\mathrm{2}}−\mathrm{1}}=\underset{\mathrm{0}\:} {\overset{\pi} {\int}}\frac{{xdx}}{\mathrm{2}\left(\mathrm{1}+\mathrm{cos}\:^{\mathrm{2}} \frac{{x}}{\mathrm{2}}\right)} \\ $$$${now}\:\mathrm{2}{I}=\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{{dx}}{\mathrm{2}\left(\mathrm{1}+{cos}^{\mathrm{2}} \frac{{x}}{\mathrm{2}}\right)} \\ $$$$=\frac{\pi}{\mathrm{2}}\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{\mathrm{sec}\:^{\mathrm{2}} \frac{{x}}{\mathrm{2}}{dx}}{\mathrm{sec}\:^{\mathrm{2}} \frac{{x}}{\mathrm{2}}+\mathrm{1}} \\ $$$$=\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{\frac{\mathrm{1}}{\mathrm{2}}\mathrm{sec}\:^{\mathrm{2}} \frac{{x}}{\mathrm{2}}{dx}}{\mathrm{2}+{tan}^{\mathrm{2}} \frac{{x}}{\mathrm{2}}} \\ $$$$=\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{{d}\left({tan}\frac{{x}}{\mathrm{2}}\right)}{\left(\sqrt{\mathrm{2}}\right)^{\mathrm{2}} +{tan}^{\mathrm{2}} \frac{{x}}{\mathrm{2}}}=\pi\left[\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\mathrm{tan}^{−\mathrm{1}} \mathrm{tan}\:\frac{{x}}{\mathrm{2}\sqrt{\mathrm{2}}}\right]_{\mathrm{0}} ^{\pi} \\ $$$$=\pi×\left[\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}×\frac{\pi}{\mathrm{2}\sqrt{\mathrm{2}}}\right]=\frac{\pi^{\mathrm{2}} }{\mathrm{4}}\therefore{I}=\frac{\pi^{\mathrm{2}} }{\mathrm{8}} \\ $$$${plz}.\:{check} \\ $$

Commented by MJS last updated on 17/May/18

in this case if f(x)=((x/2)/(1+cos^2  (x/2))) ⇒  ⇒ f(π−x)=(((π−x)/2)/(1+sin^2  (x/2)))  and we get f(x)+f(π−x)=  =((3π)/(8+sin^2  x))+((πcos x)/(8+sin^2  x))−((2xcos x)/(8+sin^2  x))  we can solve the 1^(st)  and 2^(nd)  but the 3^(rd)  looks  nasty again...

$$\mathrm{in}\:\mathrm{this}\:\mathrm{case}\:\mathrm{if}\:{f}\left({x}\right)=\frac{\frac{{x}}{\mathrm{2}}}{\mathrm{1}+\mathrm{cos}^{\mathrm{2}} \:\frac{{x}}{\mathrm{2}}}\:\Rightarrow \\ $$$$\Rightarrow\:{f}\left(\pi−{x}\right)=\frac{\frac{\pi−{x}}{\mathrm{2}}}{\mathrm{1}+\mathrm{sin}^{\mathrm{2}} \:\frac{{x}}{\mathrm{2}}} \\ $$$$\mathrm{and}\:\mathrm{we}\:\mathrm{get}\:{f}\left({x}\right)+{f}\left(\pi−{x}\right)= \\ $$$$=\frac{\mathrm{3}\pi}{\mathrm{8}+\mathrm{sin}^{\mathrm{2}} \:{x}}+\frac{\pi\mathrm{cos}\:{x}}{\mathrm{8}+\mathrm{sin}^{\mathrm{2}} \:{x}}−\frac{\mathrm{2}{x}\mathrm{cos}\:{x}}{\mathrm{8}+\mathrm{sin}^{\mathrm{2}} \:{x}} \\ $$$$\mathrm{we}\:\mathrm{can}\:\mathrm{solve}\:\mathrm{the}\:\mathrm{1}^{\mathrm{st}} \:\mathrm{and}\:\mathrm{2}^{\mathrm{nd}} \:\mathrm{but}\:\mathrm{the}\:\mathrm{3}^{\mathrm{rd}} \:\mathrm{looks} \\ $$$$\mathrm{nasty}\:\mathrm{again}... \\ $$

Commented by math1967 last updated on 17/May/18

OK thanks

$${OK}\:{thanks} \\ $$

Answered by math1967 last updated on 17/May/18

2I=∫_0 ^π ((xdx)/(3+cosx))+∫_0 ^π (((π−x)dx)/(3+cos(π−x)))  =π∫_0 ^π ((6dx)/((3+cosx)(3−cosx)))  =6π∫_0 ^π (dx/(9−cos^2 x))=6π∫_0 ^π ((sec^2 xdx)/(9sec^2 x−1))  =6π∫_0 ^π ((sec^2 xdx)/(8+9tan^2 x))=2π∫_0 ^π ((3sec^2 xdx)/((2(√(2  )))^2 +(3tan x)^2 ))  =2π∫_0 ^π ((d(3tanx))/((2(√2))^2 +(3tan x)^2 ))  =2π[(1/(2(√2)))tan^(−1) ((3tan x)/(2(√2)))]_0 ^π   =2π[(1/(2(√2)))×(π/2) −0]  =(π^2 /(2(√2))) ∴ I=(π^2 /(4(√2)))

$$\mathrm{2}{I}=\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{{xdx}}{\mathrm{3}+{cosx}}+\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{\left(\pi−{x}\right){dx}}{\mathrm{3}+{cos}\left(\pi−{x}\right)} \\ $$$$=\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{\mathrm{6}{dx}}{\left(\mathrm{3}+{cosx}\right)\left(\mathrm{3}−{cosx}\right)} \\ $$$$=\mathrm{6}\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{{dx}}{\mathrm{9}−{cos}^{\mathrm{2}} {x}}=\mathrm{6}\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{\mathrm{sec}\:^{\mathrm{2}} {xdx}}{\mathrm{9}{sec}^{\mathrm{2}} {x}−\mathrm{1}} \\ $$$$=\mathrm{6}\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{{sec}^{\mathrm{2}} {xdx}}{\mathrm{8}+\mathrm{9tan}\:^{\mathrm{2}} {x}}=\mathrm{2}\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{\mathrm{3sec}\:^{\mathrm{2}} {xdx}}{\left(\mathrm{2}\sqrt{\mathrm{2}\:\:}\right)^{\mathrm{2}} +\left(\mathrm{3tan}\:{x}\right)^{\mathrm{2}} } \\ $$$$=\mathrm{2}\pi\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{{d}\left(\mathrm{3}{tanx}\right)}{\left(\mathrm{2}\sqrt{\mathrm{2}}\right)^{\mathrm{2}} +\left(\mathrm{3tan}\:{x}\right)^{\mathrm{2}} } \\ $$$$=\mathrm{2}\pi\left[\frac{\mathrm{1}}{\mathrm{2}\sqrt{\mathrm{2}}}\mathrm{tan}^{−\mathrm{1}} \frac{\mathrm{3tan}\:{x}}{\mathrm{2}\sqrt{\mathrm{2}}}\right]_{\mathrm{0}} ^{\pi} \\ $$$$=\mathrm{2}\pi\left[\frac{\mathrm{1}}{\mathrm{2}\sqrt{\mathrm{2}}}×\frac{\pi}{\mathrm{2}}\:−\mathrm{0}\right] \\ $$$$=\frac{\pi^{\mathrm{2}} }{\mathrm{2}\sqrt{\mathrm{2}}}\:\therefore\:{I}=\frac{\pi^{\mathrm{2}} }{\mathrm{4}\sqrt{\mathrm{2}}} \\ $$

Commented by MJS last updated on 17/May/18

this seems wrong:  (x/(3+cos x))+((π−x)/(3+cos (π−x)))=(x/(3+cos x))+((π−x)/(3−cos x))=  =((3π+(π−2x)cos x)/(9−cos^2  x))≠((6π)/(9−cos^2  x))  please explain  (π^2 /(4(√2)))≈1.744716  but I get ∫_0 ^π (x/(3+cos x))dx≈1.988159

$$\mathrm{this}\:\mathrm{seems}\:\mathrm{wrong}: \\ $$$$\frac{{x}}{\mathrm{3}+\mathrm{cos}\:{x}}+\frac{\pi−{x}}{\mathrm{3}+\mathrm{cos}\:\left(\pi−{x}\right)}=\frac{{x}}{\mathrm{3}+\mathrm{cos}\:{x}}+\frac{\pi−{x}}{\mathrm{3}−\mathrm{cos}\:{x}}= \\ $$$$=\frac{\mathrm{3}\pi+\left(\pi−\mathrm{2}{x}\right)\mathrm{cos}\:{x}}{\mathrm{9}−\mathrm{cos}^{\mathrm{2}} \:{x}}\neq\frac{\mathrm{6}\pi}{\mathrm{9}−\mathrm{cos}^{\mathrm{2}} \:{x}} \\ $$$$\mathrm{please}\:\mathrm{explain} \\ $$$$\frac{\pi^{\mathrm{2}} }{\mathrm{4}\sqrt{\mathrm{2}}}\approx\mathrm{1}.\mathrm{744716} \\ $$$$\mathrm{but}\:\mathrm{I}\:\mathrm{get}\:\underset{\mathrm{0}} {\overset{\pi} {\int}}\frac{{x}}{\mathrm{3}+\mathrm{cos}\:{x}}{dx}\approx\mathrm{1}.\mathrm{988159} \\ $$

Commented by math1967 last updated on 17/May/18

Yes it is my mistake

$${Yes}\:{it}\:{is}\:{my}\:{mistake} \\ $$$$ \\ $$

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