# Quick question involving long division

• Mar 11th 2010, 04:30 PM
Archduke01
Quick question involving long division
$\displaystyle \int \frac {x^3 + 13} {x^2 + 5x + 6} dx$

The first step is to do long division. Can anyone tell me if $\displaystyle (x+5) + \frac {-19x - 17} {x^2 + 5x + 6}$ is the right answer?
• Mar 11th 2010, 04:32 PM
Miss
Quote:

Originally Posted by Archduke01
$\displaystyle \int \frac {x^3 + 13} {x^2 + 5x + 6} dx$

The first step is to do long division. Can anyone tell me if $\displaystyle (x+5) + \frac {-19x - 17} {x^2 + 5x + 6}$ is the right answer?

If you make a common denominator, do you will back to the original fraction?
• Mar 11th 2010, 04:43 PM
Archduke01
Quote:

Originally Posted by Miss
If you make a common denominator, do you will back to the original fraction?

Sorry? I don't understand your question.
• Mar 11th 2010, 05:09 PM
Quote:

Originally Posted by Archduke01
$\displaystyle \int \frac {x^3 + 13} {x^2 + 5x + 6} dx$

The first step is to do long division. Can anyone tell me if $\displaystyle (x+5) + \frac {-19x - 17} {x^2 + 5x + 6}$ is the right answer?

Hi Archduke,

you can check it quickly...

$\displaystyle x^3+13=\left(x^2+5x+6\right)(x+a)+Rem$

$\displaystyle =x^3+5x^2+6x+ax^2+5ax+6a+Rem$

$\displaystyle =x^3+(5+a)x^2+(6+5a)x+6a+Rem$

Therefore $\displaystyle a=-5$ as $\displaystyle x^3+13$ has no $\displaystyle x^2$ term.

It also has no x term, so we cancel it in the remainder

$\displaystyle x^3+13=x^3-19x-30+19x+43$

$\displaystyle \frac{x^3+13}{x^2+5x+6}=(x-5)+\frac{19x+43}{x^2+5x+6}$