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Question Number 125867 by joki last updated on 14/Dec/20

partial fraction with detail step by step from  ((4x^3 +x^2 +1)/((x^2 −2)^3 ))

partialfractionwithdetailstepbystepfrom4x3+x2+1(x22)3

Answered by mathmax by abdo last updated on 14/Dec/20

F(x)=((4x^3  +x^2  +1)/((x^2 −2)^3 )) ⇒F(x)=((4x^3  +x^2  +1)/((x−(√2))^3 (x+(√2))^3 ))  =(a/(x−(√2)))+(b/((x−(√2))^2 )) +(c/((x−(√2))^3 )) +(d/(x+(√2))) +(e/((x+(√2))^2 )) +(d/((x+(√2))^3 ))  we can know c and d    immediatly  c=(x−2)^3  F(x)∣_(x=(√2))   =((4((√2))^3  +2+1)/((2(√2))^3 ))=((8(√2)+3)/(16(√2)))  d =(x+(√2))F(x)∣_(x=−(√2))     =((4(−(√2))^3  +2+1)/((−2(√2))^3 ))   =....  lim_(x→+∞) xF(x)=0 =a+d ⇒d=−a  we can calculate F(0) ,F(1) and F(−1)to get a system after  we slve it....any way there is a lots of calculus...

F(x)=4x3+x2+1(x22)3F(x)=4x3+x2+1(x2)3(x+2)3=ax2+b(x2)2+c(x2)3+dx+2+e(x+2)2+d(x+2)3wecanknowcanddimmediatlyc=(x2)3F(x)x=2=4(2)3+2+1(22)3=82+3162d=(x+2)F(x)x=2=4(2)3+2+1(22)3=....limx+xF(x)=0=a+dd=awecancalculateF(0),F(1)andF(1)togetasystemafterweslveit....anywaythereisalotsofcalculus...

Commented by mathmax by abdo last updated on 14/Dec/20

another way by integral  we have  F(x)=(d/dx)(∫ F(x)dx)  we have ∫ F(x)dx=∫ ((4x^3  +x^2  +1)/((x^2 −2)^3 ))dx  =∫  ((4x^3  +x^2  +1)/((x−(√2))^3 (x+(√2))^3 ))dx =∫   ((4x^3  +x^2  +1)/((((x−(√2))/(x+(√2))))^3 (x+(√2))^6 ))dx  we do the changement ((x−(√2))/(x+(√2)))=t ⇒x−(√2)=tx+(√2)t ⇒  (1−t)x=(√2)t +(√2) ⇒x=(((√2)t+(√2))/(1−t)) ⇒(dx/dt)=(((√2)(1−t)+(√2)t+(√2))/((1−t)^2 ))  =((2(√2))/((t−1)^2 ))  and x+(√2)=(((√2)t+(√2))/(1−t))+(√2)=((2(√2))/(1−t)) ⇒  ∫ F(x)dx =∫   ((4((((√2)t+(√2))/(1−t)))^3  +((((√2)t+(√2))/(1−t)))^2  +1)/(t^3 (((2(√2))/(1−t)))^6 ))×((2(√2))/((t−1)^2 ))dt  =(1/((2(√2))^5 ))∫ (1/((1−t)^3 ))(4((√2)t+(√2))^3  +(1−t)((√2)t+(√2))^2 +(1−t)^3 )2(√2)(1−t)^4   (dt/t^3 )  =−(1/((2(√2))^5 ))∫  (t−1){4((√2)t+(√2))^3 +(1−t)((√2)t+(√2))^2  +(1−t)^3 )dx  its eazy to find this integral  so you get decompositino of  ∫ F(x)dx  after we derivate the integral  this method is sure...

anotherwaybyintegralwehaveF(x)=ddx(F(x)dx)wehaveF(x)dx=4x3+x2+1(x22)3dx=4x3+x2+1(x2)3(x+2)3dx=4x3+x2+1(x2x+2)3(x+2)6dxwedothechangementx2x+2=tx2=tx+2t(1t)x=2t+2x=2t+21tdxdt=2(1t)+2t+2(1t)2=22(t1)2andx+2=2t+21t+2=221tF(x)dx=4(2t+21t)3+(2t+21t)2+1t3(221t)6×22(t1)2dt=1(22)51(1t)3(4(2t+2)3+(1t)(2t+2)2+(1t)3)22(1t)4dtt3=1(22)5(t1){4(2t+2)3+(1t)(2t+2)2+(1t)3)dxitseazytofindthisintegralsoyougetdecompositinoofF(x)dxafterwederivatetheintegralthismethodissure...

Commented by joki last updated on 15/Dec/20

  thank you sir for your solution.

thankyousirforyoursolution.

Commented by mathmax by abdo last updated on 16/Dec/20

you are welcome

youarewelcome

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