Thursday, August 18, 2016

`x^2/(x^4 - 2x^2 - 8)` Write the partial fraction decomposition of the rational expression. Check your result algebraically.

`x^2/(x^4-2x^2-8)`


Let's factorize the denominator,


`x^4-2x^2-8=x^4-4x^2+2x^2-8`


`=x^2(x^2-4)+2(x^2-4)`


`=(x^2+2)(x^2-4)`


`=(x^2+2)(x+2)(x-2)`


`:.x^2/(x^4-2x^2-8)=x^2/((x+2)(x-2)(x^2+2))`


Let `x^2/((x+2)(x-2)(x^2+2))=A/(x+2)+B/(x-2)+(Cx+D)/(x^2+2)`


`=(A(x-2)(x^2+2)+B(x+2)(x^2+2)+(Cx+D)(x+2)(x-2))/((x+2)(x-2)(x^2+2))` 


`=(A(x^3+2x-2x^2-4)+B(x^3+2x+2x^2+4)+(Cx+D)(x^2-4))/((x+2)(x-2)(x^2+2))`


`=(A(x^3-2x^2+2x-4)+B(x^3+2x^2+2x+4)+Cx^3-4Cx+Dx^2-4D)/((x+2)(x-2)(x^2+2))`


`=(x^3(A+B+C)+x^2(-2A+2B+D)+x(2A+2B-4C)-4A+4B-4D)/((x+2)(x-2)(x^2+2))`


Now,


`x^2=x^3(A+B+C)+x^2(-2A+2B+D)+x(2A+2B-4C)-4A+4B-4D`


Equating the coefficients of like terms,


`A+B+C=0`     --- equation 1


`-2A+2B+D=1`   ------ equation 2


`2A+2B-4C=0`     ------ equation 3


`-4A+4B-4D=0`   ----- equation 4


Now we have to solve the above four equations to find the solutions of A,B,C and D.


Rewrite equation 1 ,


`A+B=-C`


Substitute above in equation 3,


`2(A+B)-4C=0`


`2(-C)-4C=0`


`-2C-4C=0`


`-6C=0`


`C=0`


Rewrite equation...

`x^2/(x^4-2x^2-8)`


Let's factorize the denominator,


`x^4-2x^2-8=x^4-4x^2+2x^2-8`


`=x^2(x^2-4)+2(x^2-4)`


`=(x^2+2)(x^2-4)`


`=(x^2+2)(x+2)(x-2)`


`:.x^2/(x^4-2x^2-8)=x^2/((x+2)(x-2)(x^2+2))`


Let `x^2/((x+2)(x-2)(x^2+2))=A/(x+2)+B/(x-2)+(Cx+D)/(x^2+2)`


`=(A(x-2)(x^2+2)+B(x+2)(x^2+2)+(Cx+D)(x+2)(x-2))/((x+2)(x-2)(x^2+2))` 


`=(A(x^3+2x-2x^2-4)+B(x^3+2x+2x^2+4)+(Cx+D)(x^2-4))/((x+2)(x-2)(x^2+2))`


`=(A(x^3-2x^2+2x-4)+B(x^3+2x^2+2x+4)+Cx^3-4Cx+Dx^2-4D)/((x+2)(x-2)(x^2+2))`


`=(x^3(A+B+C)+x^2(-2A+2B+D)+x(2A+2B-4C)-4A+4B-4D)/((x+2)(x-2)(x^2+2))`


Now,


`x^2=x^3(A+B+C)+x^2(-2A+2B+D)+x(2A+2B-4C)-4A+4B-4D`


Equating the coefficients of like terms,


`A+B+C=0`     --- equation 1


`-2A+2B+D=1`   ------ equation 2


`2A+2B-4C=0`     ------ equation 3


`-4A+4B-4D=0`   ----- equation 4


Now we have to solve the above four equations to find the solutions of A,B,C and D.


Rewrite equation 1 ,


`A+B=-C`


Substitute above in equation 3,


`2(A+B)-4C=0`


`2(-C)-4C=0`


`-2C-4C=0`


`-6C=0`


`C=0`


Rewrite equation 2 as,


`-2A+2B=1-D`


Substitute the above in equation 4,


`-4A+4B-4D=0`  


`2(-2A+2B)-4D=0`


`2(1-D)-4D=0`


`2-2D-4D=0`


`2-6D=0`


`D=2/6`


`D=1/3`


Plug the values of C in equation 3 ,


`2A+2B-4(0)=0`


`2A+2B=0`


`2(A+B)=0`


`A+B=0`    ------ equation 5


Plug the value of D in equation 4,


`-4A+4B-4(1/3)=0`


`-4A+4B=4/3`


`4(-A+B)=4/3`


`-A+B=1/3`        ---- equation 6


Solve equations 5 and 6 to find the solutions of A and B,


Add the equations 5 and 6.


`2B=1/3`  


`B=1/6`


Plug the value of B in equation 5,


`A+1/6=0`


`A=-1/6`


`:.x^2/(x^4-2x^2-8)=(-1/6)/(x+2)+(1/6)/(x-2)+(0*x+1/3)/(x^2+2)`


`x^2/(x^4-2x^2-8)=-1/(6(x+2))+1/(6(x-2))+1/(3(x^2+2))`

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