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Question Number 106948 by abdomathmax last updated on 08/Aug/20

let f(a) =∫_0 ^π  (dx/(a+cos^2 x)) with a>0  1) explicite f(a)  2)explicite g(a) =∫_0 ^π   (dx/((a+cos^2 x)^2 ))  3) find tbe valued of intevrsls  ∫_0 ^π  (dx/(1+cos^2 x)) and ∫_0 ^π  (dx/((1+cos^2 x)^2 ))

letf(a)=0πdxa+cos2xwitha>0 1)explicitef(a) 2)expliciteg(a)=0πdx(a+cos2x)2 3)findtbevaluedofintevrsls 0πdx1+cos2xand0πdx(1+cos2x)2

Answered by Ar Brandon last updated on 08/Aug/20

f(a)=∫_0 ^π (dx/(a+cos^2 x))=∫_0 ^π ((sec^2 x)/(asec^2 x+1))dx=∫_0 ^π ((d(tanx))/(a(1+tan^2 x)+1))           =∫_0 ^π ((d(tanx))/((a+1)+atan^2 x))=(1/a)∫_0 ^π ((d(tanx))/((((a+1)/a))+tan^2 x))           =(1/a)∙(√(a/(a+1)))[Arctan((√(a/(a+1)))∙tanx)]_0 ^π            =(π/a)∙(√(a/(a+1)))=(π/(√(a^2 +a)))

f(a)=0πdxa+cos2x=0πsec2xasec2x+1dx=0πd(tanx)a(1+tan2x)+1 =0πd(tanx)(a+1)+atan2x=1a0πd(tanx)(a+1a)+tan2x =1aaa+1[Arctan(aa+1tanx)]0π =πaaa+1=πa2+a

Answered by Ar Brandon last updated on 08/Aug/20

g(a)=∫_0 ^π (dx/((a+cos^2 x)^2 ))=∫_0 ^π ((sec^4 x)/((asec^2 x+1)^2 ))dx            =∫_0 ^π ((sec^2 x∙d(tanx))/((a+atan^2 x+1)^2 ))=∫_0 ^π ((1+tan^2 x)/((a+1+atan^2 x)^2 ))d(tanx)            =∫_0 ^(−0) ((1+t^2 )/((a+1+at^2 )^2 ))dt=∫_0 ^(−0) ((1+t^2 )/((a+1)^2 +2a(a+1)+a^2 t^4 ))dt            =(1/(2a))∫_0 ^(−0) (((a+1+at^2 )−(a+1−at^2 ))/((a+1)^2 +2a(a+1)+a^2 t^4 ))dt+∫_0 ^(−0) (1/((a+1)^2 +2a(a+1)+a^2 t^4 ))dt            =(1/(2a)){∫_0 ^(−0) (((a+1)+at^2 )/((a+1)^2 +2a(a+1)+a^2 t^4 ))dt                −∫_0 ^(−0) (((a+1)−at^2 )/((a+1)^2 +2a(a+1)+a^2 t^4 ))dt {: (),() }+I             =(1/(2a)){∫_0 ^(−0) ((1+((a+1)/(at^2 )))/((((a+1)/(at)))^2 +((2(a+1))/t^2 )+at^2 ))+∫_0 ^(−0) ((1−((a+1)/(at^2 )))/((((a+1)/(at)))^2 +((2(a+1))/t^2 )+at^2 ))dt}+I  ...

g(a)=0πdx(a+cos2x)2=0πsec4x(asec2x+1)2dx =0πsec2xd(tanx)(a+atan2x+1)2=0π1+tan2x(a+1+atan2x)2d(tanx) =001+t2(a+1+at2)2dt=001+t2(a+1)2+2a(a+1)+a2t4dt =12a00(a+1+at2)(a+1at2)(a+1)2+2a(a+1)+a2t4dt+001(a+1)2+2a(a+1)+a2t4dt =12a{00(a+1)+at2(a+1)2+2a(a+1)+a2t4dt 00(a+1)at2(a+1)2+2a(a+1)+a2t4dt}+I =12a{001+a+1at2(a+1at)2+2(a+1)t2+at2+001a+1at2(a+1at)2+2(a+1)t2+at2dt}+I ...

Answered by Ar Brandon last updated on 08/Aug/20

f(1)=∫_0 ^π (dx/(1+cos^2 x))=(π/(√(1^2 +1)))=(π/(√2))  g(1)=∫_0 ^π (dx/((1+cos^2 x)^2 ))=∫_0 ^π ((sec^4 x)/((sec^2 x+1)^2 ))dx           =∫_0 ^π ((1+tan^2 x)/((2+tan^2 x)^2 ))d(tanx)=∫_0 ^(−0) ((1+t^2 )/((2+t^2 )^2 ))dt           =∫_0 ^(−0) ((2+t^2 )/((2+t^2 )^2 ))dt−∫_0 ^(−0) (1/((2+t^2 )^2 ))dt     =_(t=(√2)tanθ) ∫_0 ^(−0) (dt/(2+t^2 ))−(1/4)∫_0 ^π (((√2)sec^2 θdθ)/((1+tan^2 θ)^2 ))          =[(1/(√2))Arctan((t/(√2)))]_0 ^(−0) −((√2)/4)∫_0 ^π (dθ/(sec^2 θ))          =(π/(√2))−((√2)/4)∙(1/2)∫_0 ^π (1+cos2θ)dθ=(π/(√2))−(1/(4(√2)))[θ+((sin2θ)/2)]_0 ^π           =(π/(√2))−(π/(4(√2)))=((3π)/(4(√2)))

f(1)=0πdx1+cos2x=π12+1=π2 g(1)=0πdx(1+cos2x)2=0πsec4x(sec2x+1)2dx =0π1+tan2x(2+tan2x)2d(tanx)=001+t2(2+t2)2dt =002+t2(2+t2)2dt001(2+t2)2dt =t=2tanθ00dt2+t2140π2sec2θdθ(1+tan2θ)2 =[12Arctan(t2)]00240πdθsec2θ =π224120π(1+cos2θ)dθ=π2142[θ+sin2θ2]0π =π2π42=3π42

Commented byAr Brandon last updated on 08/Aug/20

You're welcome, senior.��

Commented byabdomathmax last updated on 08/Aug/20

thank you sir brandon

thankyousirbrandon

Answered by mathmax by abdo last updated on 08/Aug/20

1) residus method f(a) =∫_0 ^π  (dx/(a+cos^2 x)) ⇒f(a) =∫_0 ^π  (dx/(a+((1+cos(2x))/2)))  =2 ∫_0 ^π  (dx/(2a+1+cos(2x))) =_(2x=t)    2 ∫_0 ^(2π)   (dt/(2(2a+1+cost))) =∫_0 ^(2π)  (dt/(2a+1+cost))  =_(e^(it)  =z)     ∫_(∣z∣=1)        (dz/(iz(2a+1+((z+z^(−1) )/2)))) =∫ ((−2idz)/(z(4a+2+z+z^(−1) )))  =∫  ((−2idz)/((4a+2)z +z^2  +1))  let ϕ(z) =((−2i)/(z^2  +(4a+2)z +1))  poles of ϕ?  Δ^′  =(2a+1)^2 −1 =4a^2 +4a+1−1 =4a^2  +4a ⇒  z_1 =−(2a+1)+2(√(a^2  +a))  and z_2 =−(2a+1)−2(√(a^2  +a))  ∣z_1 ∣ −1 =∣2(√(a^2 +a))−(2a+1)∣−1 =2a+1−2(√(a^2 +a))−1  =2a−2(√(a^2 +a))=2(a−(√(a^2 +a)))<0 ⇒∣z_1 ∣<1  ∣z_2 ∣−1 =2a+1+2(√(a^2  +a))−1 >0 ⇒∣z_2 ∣>1(to eliminate from residus)  ⇒ ∫_(∣z∣=1)    ϕ(2)dz =2iπ Res(ϕ,z_1 ) =2iπ×((−2i)/(z_1 −z_2 ))  =((4π)/(4(√(a^2  +a)))) =(π/(√(a^2  +a))) ⇒f(a) =(π/(√(a^2  +a)))   (a>0)  2)we have f^′ (a) =−∫_0 ^π   (dx/((a+cos^2 x)^2 )) =−g(a) ⇒  g(a) =−f^′ (a)  but  f^′ (a) =−π ×((((√(a^2 +a)))^′ )/(a^2  +a⇒)) =−π×((2a+1)/(2(a^2  +a)(√(a^2 +a))))  ⇒g(a) =(((2a+1)π)/(2(a^2  +a)(√(a^2  +a))))  3) ∫_0 ^π   (dx/(1+cos^2 x)) =f(1) =(π/(√2))  ∫_0 ^π  (dx/((1+cos^2 x)^2 )) =g(1) =((3π)/(4(√2)))

1)residusmethodf(a)=0πdxa+cos2xf(a)=0πdxa+1+cos(2x)2 =20πdx2a+1+cos(2x)=2x=t202πdt2(2a+1+cost)=02πdt2a+1+cost =eit=zz∣=1dziz(2a+1+z+z12)=2idzz(4a+2+z+z1) =2idz(4a+2)z+z2+1letφ(z)=2iz2+(4a+2)z+1polesofφ? Δ=(2a+1)21=4a2+4a+11=4a2+4a z1=(2a+1)+2a2+aandz2=(2a+1)2a2+a z11=∣2a2+a(2a+1)1=2a+12a2+a1 =2a2a2+a=2(aa2+a)<0⇒∣z1∣<1 z21=2a+1+2a2+a1>0⇒∣z2∣>1(toeliminatefromresidus) z∣=1φ(2)dz=2iπRes(φ,z1)=2iπ×2iz1z2 =4π4a2+a=πa2+af(a)=πa2+a(a>0) 2)wehavef(a)=0πdx(a+cos2x)2=g(a) g(a)=f(a)butf(a)=π×(a2+a)a2+a=π×2a+12(a2+a)a2+a g(a)=(2a+1)π2(a2+a)a2+a 3)0πdx1+cos2x=f(1)=π2 0πdx(1+cos2x)2=g(1)=3π42

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