Integration of Rational Functions By Partial Fractions

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June 21st 2008, 09:41 PM
auslmar

Integration of Rational Functions By Partial Fractions

I have this problem that needs to be solved by integration by partial fractions and I can't seem to get the right answer. Here's the problem as written:

"Find $\int\frac{6x+1}{20x^2+14x-12}dx$ with respect to $x$ , use the constant $C$ ."

So, I have:

$ \int\frac{6x+1}{(5x+6)(4x-2)}dx = \int[\frac{A}{5x+6} + \frac{B}{4x-2}]dx $

and

$ 6x+1 = A(4x-2) + B(5x+6) $

If $x=\frac{1}{2}$ , then $B = \frac{8}{17}$

and

If $x = \frac{-6}{5}$ , then $A = \frac{31}{850}$

So,

$ \int[\frac{\frac{-41}{34}}{5x+6} + \frac{\frac{8}{17}}{4x-2}]dx$

$ = \frac{-41}{34}ln(5x+6) + \frac{8}{17}ln(4x-2) + C $

Unfortunately, this is not the answer. I greatly appreciate if anyone can point out where I've gone wrong. I have a feeling it was a generally small, but ultimately large, error. It is entirely possible that I have made an arthimetic mistake, by the way.

Thanks for your consideration,

Austin Martin
June 21st 2008, 09:47 PM
mr fantastic

Quote:

Originally Posted by auslmar

I have this problem that needs to be solved by integration by partial fractions and I can't seem to get the right answer. Here's the problem as written:

"Find $\int\frac{6x+1}{20x^2+14x-12}dx$ with respect to $x$ , use the constant $C$ ."

So, I have:

$ \int\frac{6x+1}{(5x+6)(4x-2)}dx = \int[\frac{A}{5x+6} + \frac{B}{4x-2}]dx $

and

$ 6x+1 = A(4x-2) + B(5x+6) $

If $x=\frac{1}{2}$ , then $B = \frac{8}{17}$

and

If $x = \frac{-6}{5}$ , then $A = \frac{31}{850}$

So,

$ \int[\frac{\frac{-41}{34}}{5x+6} + \frac{\frac{8}{17}}{4x-2}]dx$

$ = \frac{-41}{34}ln(5x+6) + \frac{8}{17}ln(4x-2) + C $

Unfortunately, this is not the answer. I greatly appreciate if anyone can point out where I've gone wrong. I have a feeling it was a generally small, but ultimately large, error. It is entirely possible that I have made an arthimetic mistake, by the way.

Thanks for your consideration,

Austin Martin

B is OK. A is wrong .... A = 31/34.
June 21st 2008, 09:53 PM
mr fantastic

Quote:

Originally Posted by mr fantastic

B is correct. A is wrong .... A = 31/34.

So the answer becomes

${\color{red}\frac{1}{5}} \, \frac{31}{34} \, \ln{\color{red}|} 5x + 6{\color{red}|} + {\color{red}\frac{1}{4}} \, \frac{{\color{red}8}}{17} \, \ln {\color{red}|} 4x - 2 {\color{red}|} + C = {\color{red}\frac{1}{5}} \, \frac{31}{34} \, \ln{\color{red} |} 5x + 6 {\color{red}|} + {\color{red}\frac{1}{4}} \, \frac{{\color{red}16}}{34} \, \ln {\color{red}|} 4x - 2 {\color{red}|} + C = .....$

where you keep re-arranging until you recognise the correct answer.

Note the use of absolute value!
June 21st 2008, 09:54 PM
Mathstud28

Quote:

Originally Posted by auslmar

I have this problem that needs to be solved by integration by partial fractions and I can't seem to get the right answer. Here's the problem as written:

"Find $\int\frac{6x+1}{20x^2+14x-12}dx$ with respect to $x$ , use the constant $C$ ."

So, I have:

$ \int\frac{6x+1}{(5x+6)(4x-2)}dx = \int[\frac{A}{5x+6} + \frac{B}{4x-2}]dx $

and

$ 6x+1 = A(4x-2) + B(5x+6) $

If $x=\frac{1}{2}$ , then $B = \frac{8}{17}$

and

If $x = \frac{-6}{5}$ , then $A = \frac{31}{850}$

So,

$ \int[\frac{\frac{-41}{34}}{5x+6} + \frac{\frac{8}{17}}{4x-2}]dx$

$ = \frac{-41}{34}ln(5x+6) + \frac{8}{17}ln(4x-2) + C $

Unfortunately, this is not the answer. I greatly appreciate if anyone can point out where I've gone wrong. I have a feeling it was a generally small, but ultimately large, error. It is entirely possible that I have made an arthimetic mistake, by the way.

Thanks for your consideration,

Austin Martin

If you need to I will go through the entire PFD for you, but a first thing that will make it a little bit easier. Why not finish your factorization?

$4x-2=2(2x-1)$

So you can forego it
June 21st 2008, 10:38 PM
auslmar

Thanks to both of you. Now that it's established that I suck at arithmetic, I'm having trouble seeing why the answer is not:

$ \frac{31}{34}ln|5x+6| + \frac{8}{17}ln|4x-2| + C $ since $A = \frac{31}{34}$ and $B = \frac{8}{17}$
Also Mr. Fantastic, I'm not sure what you mean by:

Quote:

Originally Posted by mr fantastic

So the answer becomes

$\frac{31}{34} \, \ln{\color{red}|} 5x + 6{\color{red}|} + \frac{4}{17} \, \ln {\color{red}|} 4x - 2 {\color{red}|} + C = \frac{31}{34} \, \ln{\color{red} |} 5x + 6 {\color{red}|} + \frac{8}{34} \, \ln {\color{red}|} 4x - 2 {\color{red}|} + C = .....$

where you keep re-arranging until you recognise the correct answer.

Note the use of absolute value!

Re-arranging? Forgive my dense-ness.
June 21st 2008, 11:59 PM
mr fantastic

Quote:

Originally Posted by auslmar

Thanks to both of you. Now that it's established that I suck at arithmetic, I'm having trouble seeing why the answer is not:

$ \frac{31}{34}ln|5x+6| + \frac{8}{17}ln|4x-2| + C $ since $A = \frac{31}{34}$ and $B = \frac{8}{17}$

Mr F says: That's one form of the correct answer.

Also Mr. Fantastic, I'm not sure what you mean by:

Re-arranging? Forgive my dense-ness.

Other forms of the correct answer can be found by re-arranging the above. For example:

$\frac{1}{34} \left( 31 \ln|5x+6| + 16 \ln|4x-2| + C \right)$ .

If you state what form the final answer is given in, you can be shown how to get that form from the above.
June 22nd 2008, 09:33 AM
auslmar

Quote:

Originally Posted by mr fantastic

Other forms of the correct answer can be found by re-arranging the above. For example:

$\frac{1}{34} \left( 31 \ln|5x+6| + 16 \ln|4x-2| + C \right)$ .

If you state what form the final answer is given in, you can be shown how to get that form from the above.

Oh, I see. I use a homework system called Maple TA that allows the user to preview if their answer is correct. Unfortunately, when I enter any forms of the previously determined correct answer, the system says they're wrong. I can't figure out what's wrong. I'm using absolute values and including the costant $C$ , so I'm puzzled as to why it's not recognizing the answer.
June 22nd 2008, 12:59 PM
mr fantastic

Quote:

Originally Posted by mr fantastic

So the answer becomes

${\color{red}\frac{1}{5}} \, \frac{31}{34} \, \ln{\color{red}|} 5x + 6{\color{red}|} + {\color{red}\frac{1}{4}} \, \frac{4}{17} \, \ln {\color{red}|} 4x - 2 {\color{red}|} + C = {\color{red}\frac{1}{5}} \, \frac{31}{34} \, \ln{\color{red} |} 5x + 6 {\color{red}|} + {\color{red}\frac{1}{4}} \, \frac{{\color{red}16}}{34} \, \ln {\color{red}|} 4x - 2 {\color{red}|} + C = .....$

where you keep re-arranging until you recognise the correct answer.

Note the use of absolute value!

My mistake. See the edit above. I was very careless. So were you, of course.

In fact, if you follow mathstud's suggestion of simplifying 4x - 2 = 2(x - 1), you will probably get the form of the answer that's being looked for a lot easier ......

Note that $\frac{1}{4} \, \frac{16}{34} \, \ln | 4x - 2 | + C = \frac{4}{34} \, \ln 2| 2x - 1 | + C$

$= \frac{4}{34} \, \ln| 2x - 1 | + \frac{4}{34} \, \ln (2) + C$ where the last is just a constant, D say

$= \frac{4}{34} \, \ln| 2x - 1 | + D = \frac{20}{170} \, \ln| 2x - 1 | + D$

That should give you blue sky .........