Sunday, June 30, 2013

`(x^2 + 5)/((x + 1)(x^2 - 2x + 3))` Write the partial fraction decomposition of the rational expression. Check your result algebraically.

`(x^2+5)/((x+1)(x^2-2x+3))` 


Let`(x^2+5)/((x+1)(x^2-2x+3))=A/(x+1)+(Bx+C)/(x^2-2x+3)`


`(x^2+5)/((x+1)(x^2-2x+3))=(A(x^2-2x+3)+(Bx+C)(x+1))/((x+1)(x^2-2x+3))`


`(x^2+5)/((x+1)(x^2-2x+3))=(Ax^2-2Ax+3A+Bx^2+Bx+Cx+C)/((x+1)(x^2-2x+3))`


`:.(x^2+5)=Ax^2-2Ax+3A+Bx^2+Bx+Cx+C`


`x^2+5=(A+B)x^2+(-2A+B+C)x+3A+C`


equating the coefficients of the like terms,


`A+B=1`


`-2A+B+C=0`


`3A+C=5`


Now let's solve the above three equations to find the values of A,B and C,


Express C in terms of A from the third equation,


`C=5-3A`


Substitute the above expression of C in second equation,


`-2A+B+5-3A=0`


`-5A+B+5=0`


`-5A+B=-5`


Now subtract the first equation from the above equation,


`(-5A+B)-(A+B)=-5-1`


`-6A=-6`


`A=1`


Plug the value of A in the first and...

`(x^2+5)/((x+1)(x^2-2x+3))` 


Let`(x^2+5)/((x+1)(x^2-2x+3))=A/(x+1)+(Bx+C)/(x^2-2x+3)`


`(x^2+5)/((x+1)(x^2-2x+3))=(A(x^2-2x+3)+(Bx+C)(x+1))/((x+1)(x^2-2x+3))`


`(x^2+5)/((x+1)(x^2-2x+3))=(Ax^2-2Ax+3A+Bx^2+Bx+Cx+C)/((x+1)(x^2-2x+3))`


`:.(x^2+5)=Ax^2-2Ax+3A+Bx^2+Bx+Cx+C`


`x^2+5=(A+B)x^2+(-2A+B+C)x+3A+C`


equating the coefficients of the like terms,


`A+B=1`


`-2A+B+C=0`


`3A+C=5`


Now let's solve the above three equations to find the values of A,B and C,


Express C in terms of A from the third equation,


`C=5-3A`


Substitute the above expression of C in second equation,


`-2A+B+5-3A=0`


`-5A+B+5=0`


`-5A+B=-5`


Now subtract the first equation from the above equation,


`(-5A+B)-(A+B)=-5-1`


`-6A=-6`


`A=1`


Plug the value of A in the first and third equation to get the values of B and C,


`1+B=1`  


`B=1-1`


`B=0`


`3(1)+C=5`


`C=5-3`


`C=2`


`:.(x^2+5)/((x+1)(x^2-2x+3))=1/(x+1)+2/(x^2-2x+3)`


Now let's check it algebraically,


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


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


`=(x^2+5)/((x+1)(x^2-2x+3))`


Hence it is verified.

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