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Question Number 38113 by maxmathsup by imad last updated on 21/Jun/18

let p>1 calculate  ∫_0 ^(2π)       (dt/((p +cost)^2 ))

$${let}\:{p}>\mathrm{1}\:{calculate}\:\:\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\:\:\:\:\:\frac{{dt}}{\left({p}\:+{cost}\right)^{\mathrm{2}} } \\ $$

Commented bymath khazana by abdo last updated on 08/Jul/18

let put A_p = ∫_0 ^(2π)      (dt/((p +cost)^2 ))  changement  e^(it)  =z give  A_p = ∫_(∣z∣=1)     (1/((p +((z+z^(−1) )/2))^2 )) (dz/(iz))  =∫_(∣z∣=1)       ((−4i dz)/(z(2p +z +z^(−1) )^2 ))  = ∫_(∣z∣=1)     ((−4idz)/(z( 2p +z +(1/z))^2 ))  =∫_(∣z∣=1)      ((−4izdz)/((2pz +z^2  +1)^2 ))  =∫_(∣z∣=1)      ((−4izdz)/((z^2  +2pz +1)^2 ))  let  ϕ(z) = ((−4iz)/((z^2  +2pz +1)^2 )) poles of ϕ?  roots of z^2  +2pz +1  Δ^′  =p^2 −1  >0 ⇒ z_1 =−p +(√(p^2  −1))  z_2 =−p −(√(p^2  −1))  ϕ(z)   =  ((−4iz)/((z−z_1 )^2 (z−z_2 )^2 ))  ∣z_1 ∣ −1 = p−(√(p^2 −1))−1 =p−1 −(√(p^2  −1))  (p−1)^2 −(p^2 −1) =p^2  −2p +1−p^2  +1  =−2p+2 =−2(p−1)<0 ⇒∣z_1 ∣<1  ∣z_2 ∣>1 ⇒ ∫_(∣z∣=1) ϕ(z)dz =2iπ Res(ϕ,z_1 )

$${let}\:{put}\:{A}_{{p}} =\:\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\:\:\:\:\frac{{dt}}{\left({p}\:+{cost}\right)^{\mathrm{2}} }\:\:{changement} \\ $$ $${e}^{{it}} \:={z}\:{give} \\ $$ $${A}_{{p}} =\:\int_{\mid{z}\mid=\mathrm{1}} \:\:\:\:\frac{\mathrm{1}}{\left({p}\:+\frac{{z}+{z}^{−\mathrm{1}} }{\mathrm{2}}\right)^{\mathrm{2}} }\:\frac{{dz}}{{iz}} \\ $$ $$=\int_{\mid{z}\mid=\mathrm{1}} \:\:\:\:\:\:\frac{−\mathrm{4}{i}\:{dz}}{{z}\left(\mathrm{2}{p}\:+{z}\:+{z}^{−\mathrm{1}} \right)^{\mathrm{2}} } \\ $$ $$=\:\int_{\mid{z}\mid=\mathrm{1}} \:\:\:\:\frac{−\mathrm{4}{idz}}{{z}\left(\:\mathrm{2}{p}\:+{z}\:+\frac{\mathrm{1}}{{z}}\right)^{\mathrm{2}} } \\ $$ $$=\int_{\mid{z}\mid=\mathrm{1}} \:\:\:\:\:\frac{−\mathrm{4}{izdz}}{\left(\mathrm{2}{pz}\:+{z}^{\mathrm{2}} \:+\mathrm{1}\right)^{\mathrm{2}} } \\ $$ $$=\int_{\mid{z}\mid=\mathrm{1}} \:\:\:\:\:\frac{−\mathrm{4}{izdz}}{\left({z}^{\mathrm{2}} \:+\mathrm{2}{pz}\:+\mathrm{1}\right)^{\mathrm{2}} }\:\:{let} \\ $$ $$\varphi\left({z}\right)\:=\:\frac{−\mathrm{4}{iz}}{\left({z}^{\mathrm{2}} \:+\mathrm{2}{pz}\:+\mathrm{1}\right)^{\mathrm{2}} }\:{poles}\:{of}\:\varphi? \\ $$ $${roots}\:{of}\:{z}^{\mathrm{2}} \:+\mathrm{2}{pz}\:+\mathrm{1} \\ $$ $$\Delta^{'} \:={p}^{\mathrm{2}} −\mathrm{1}\:\:>\mathrm{0}\:\Rightarrow\:{z}_{\mathrm{1}} =−{p}\:+\sqrt{{p}^{\mathrm{2}} \:−\mathrm{1}} \\ $$ $${z}_{\mathrm{2}} =−{p}\:−\sqrt{{p}^{\mathrm{2}} \:−\mathrm{1}} \\ $$ $$\varphi\left({z}\right)\:\:\:=\:\:\frac{−\mathrm{4}{iz}}{\left({z}−{z}_{\mathrm{1}} \right)^{\mathrm{2}} \left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{2}} } \\ $$ $$\mid{z}_{\mathrm{1}} \mid\:−\mathrm{1}\:=\:{p}−\sqrt{{p}^{\mathrm{2}} −\mathrm{1}}−\mathrm{1}\:={p}−\mathrm{1}\:−\sqrt{{p}^{\mathrm{2}} \:−\mathrm{1}} \\ $$ $$\left({p}−\mathrm{1}\right)^{\mathrm{2}} −\left({p}^{\mathrm{2}} −\mathrm{1}\right)\:={p}^{\mathrm{2}} \:−\mathrm{2}{p}\:+\mathrm{1}−{p}^{\mathrm{2}} \:+\mathrm{1} \\ $$ $$=−\mathrm{2}{p}+\mathrm{2}\:=−\mathrm{2}\left({p}−\mathrm{1}\right)<\mathrm{0}\:\Rightarrow\mid{z}_{\mathrm{1}} \mid<\mathrm{1} \\ $$ $$\mid{z}_{\mathrm{2}} \mid>\mathrm{1}\:\Rightarrow\:\int_{\mid{z}\mid=\mathrm{1}} \varphi\left({z}\right){dz}\:=\mathrm{2}{i}\pi\:{Res}\left(\varphi,{z}_{\mathrm{1}} \right) \\ $$ $$ \\ $$

Commented bymath khazana by abdo last updated on 08/Jul/18

Res(ϕ,z_1 ) =lim_(z→z_1 )   ?(1/((2−1)!)) {(z−z_1 )^2  ϕ(z)}^((1))   =lim_(z→z_1 )    { ((−4iz)/((z−z_2 )^2 ))}^((1))   =−4i lim_(z→z_1 ) (((z−z_2 )^2  −2(z−z_2 )z)/((z−z_2 )^4 ))  =−4i lim_(z→z_1 )  (((z−z_2 ) −2z)/((z−z_2 )^3 ))  =4i lim_(z→z_1 )   ((z +z_2 )/((z−z_2 )^2 ))  =4i ((z_1  +z_2 )/((z_(1 ) −z_2 )^2 )) =4i  ((−2p)/((2(√(p^2 −1)))^2 )) = ((−2ip)/((p^2  −1)))  ∫_(−∞) ^(+∞)   ϕ(z)dz =2iπ  (((−2ip)/(p^2 −1)))= ((4pπ)/(p^2 −1)) ⇒  A_p = ((4pπ)/(p^2  −1)) .

$${Res}\left(\varphi,{z}_{\mathrm{1}} \right)\:={lim}_{{z}\rightarrow{z}_{\mathrm{1}} } \:\:?\frac{\mathrm{1}}{\left(\mathrm{2}−\mathrm{1}\right)!}\:\left\{\left({z}−{z}_{\mathrm{1}} \right)^{\mathrm{2}} \:\varphi\left({z}\right)\right\}^{\left(\mathrm{1}\right)} \\ $$ $$={lim}_{{z}\rightarrow{z}_{\mathrm{1}} } \:\:\:\left\{\:\frac{−\mathrm{4}{iz}}{\left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{2}} }\right\}^{\left(\mathrm{1}\right)} \\ $$ $$=−\mathrm{4}{i}\:{lim}_{{z}\rightarrow{z}_{\mathrm{1}} } \frac{\left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{2}} \:−\mathrm{2}\left({z}−{z}_{\mathrm{2}} \right){z}}{\left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{4}} } \\ $$ $$=−\mathrm{4}{i}\:{lim}_{{z}\rightarrow{z}_{\mathrm{1}} } \:\frac{\left({z}−{z}_{\mathrm{2}} \right)\:−\mathrm{2}{z}}{\left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{3}} } \\ $$ $$=\mathrm{4}{i}\:{lim}_{{z}\rightarrow{z}_{\mathrm{1}} } \:\:\frac{{z}\:+{z}_{\mathrm{2}} }{\left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{2}} } \\ $$ $$=\mathrm{4}{i}\:\frac{{z}_{\mathrm{1}} \:+{z}_{\mathrm{2}} }{\left({z}_{\mathrm{1}\:} −{z}_{\mathrm{2}} \right)^{\mathrm{2}} }\:=\mathrm{4}{i}\:\:\frac{−\mathrm{2}{p}}{\left(\mathrm{2}\sqrt{{p}^{\mathrm{2}} −\mathrm{1}}\right)^{\mathrm{2}} }\:=\:\frac{−\mathrm{2}{ip}}{\left({p}^{\mathrm{2}} \:−\mathrm{1}\right)} \\ $$ $$\int_{−\infty} ^{+\infty} \:\:\varphi\left({z}\right){dz}\:=\mathrm{2}{i}\pi\:\:\left(\frac{−\mathrm{2}{ip}}{{p}^{\mathrm{2}} −\mathrm{1}}\right)=\:\frac{\mathrm{4}{p}\pi}{{p}^{\mathrm{2}} −\mathrm{1}}\:\Rightarrow \\ $$ $${A}_{{p}} =\:\frac{\mathrm{4}{p}\pi}{{p}^{\mathrm{2}} \:−\mathrm{1}}\:. \\ $$

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