Friday, August 16, 2013

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

`3/(x^4+x)`


Let's factorize the denominator,


`x^4+x=x(x^3+1)`


`=x(x+1)(x^2-x+1)`


Let `3/(x^4+x)=A/x+B/(x+1)+(Cx+D)/(x^2-x+1)`


`3/(x^4+x)=(A(x+1)(x^2-x+1)+B(x)(x^2-x+1)+(Cx+D)(x)(x+1))/(x(x+1)(x^2-x+1))`


`3/(x^4+x)=(A(x^3-x^2+x+x^2-x+1)+B(x^3-x^2+x)+(Cx+D)(x^2+x))/(x(x+1)(x^2-x+1))`


`3/(x^4+x)=(A(x^3+1)+B(x^3-x^2+x)+Cx^3+Cx^2+Dx^2+Dx)/(x(x+1)(x^2-x+1))`


`3/(x^4+x)=(x^3(A+B+C)+x^2(-B+C+D)+x(B+D)+A)/(x(x+1)(x^2-x+1))`


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


equating the coefficients of the like terms,


`A+B+C=0`       - equation 1


`-B+C+D=0`    - equation 2


`B+D=0`              - equation 3


`A=3`


Plug the value of A in equation 1,


`3+B+C=0`


`B+C=-3`


`C=-3-B`


Substitute the above expression of C in equation 2,


`-B+(-3-B)+D=0`


`-B-3-B+D=0`


`-2B+D=3`      - equation 4


Now solve equations 3 and...

`3/(x^4+x)`


Let's factorize the denominator,


`x^4+x=x(x^3+1)`


`=x(x+1)(x^2-x+1)`


Let `3/(x^4+x)=A/x+B/(x+1)+(Cx+D)/(x^2-x+1)`


`3/(x^4+x)=(A(x+1)(x^2-x+1)+B(x)(x^2-x+1)+(Cx+D)(x)(x+1))/(x(x+1)(x^2-x+1))`


`3/(x^4+x)=(A(x^3-x^2+x+x^2-x+1)+B(x^3-x^2+x)+(Cx+D)(x^2+x))/(x(x+1)(x^2-x+1))`


`3/(x^4+x)=(A(x^3+1)+B(x^3-x^2+x)+Cx^3+Cx^2+Dx^2+Dx)/(x(x+1)(x^2-x+1))`


`3/(x^4+x)=(x^3(A+B+C)+x^2(-B+C+D)+x(B+D)+A)/(x(x+1)(x^2-x+1))`


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


equating the coefficients of the like terms,


`A+B+C=0`       - equation 1


`-B+C+D=0`    - equation 2


`B+D=0`              - equation 3


`A=3`


Plug the value of A in equation 1,


`3+B+C=0`


`B+C=-3`


`C=-3-B`


Substitute the above expression of C in equation 2,


`-B+(-3-B)+D=0`


`-B-3-B+D=0`


`-2B+D=3`      - equation 4


Now solve equations 3 and 4 to get the solutions of B and D,


Subtract equation 3 from equation 4,


`(-2B+D)-(B+D)=3-0`


`-3B=3`


`B=-1`


Plug the value of B in equation 3,


`-1+D=0`


`D=1`


Plug the value of A and B in equation 1,


`3+(-1)+C=0`


`2+C=0`


`C=-2`


`:.3/(x^4+x)=3/x-1/(x+1)+(-2x+1)/(x^2-x+1)`


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