Integrate: (x^2 + 4)/ (x^2+5x-6) dx

1. Nov 10, 2012

beaf123

1. The problem statement, all variables and given/known data

"Express the integrand (what does "integrand" mean?) as a sum of partial fractions and evaluate the integrals.

∫(x + 4)/ (x^2+5x-6) dx

2. Relevant equations

3. The attempt at a solution

x^2+5x-6 = (x-1)(x+6)

Gives:

∫ A/(x-1) + B/(x+6) dx

Findig A and B:

A(x+6) + B(x-1)

A+B=1

6A-B =4

A= 5/7
B= - (2/7)

Then:

∫ (5/7) (1/x-1) dx +∫ - (3/7)(1/x+6) dx

Gives I think:

5/7ln(x-1) -2/7ln(x+6)

But its wrong accordng to the solution. I can post the solution here if you want me to:

Last edited: Nov 10, 2012
2. Nov 10, 2012

Mentallic

The integrand is the expression being integrated, in this case, the rational function $$\frac{x+4}{x^2+5x-6}$$

How did you get A+B=1 ?

3. Nov 10, 2012

beaf123

Okey, thanks.

A(x+6) + B(x-1) = x+4

Ax +Bx +6A -B = x+4

Ax + Bx = x

A+B =1

4. Nov 10, 2012

Mentallic

Oh I see, you were comparing coefficients

Sorry I didn't spot your error before, but you incorrectly solved the simultaneous equations in A and B. 1 - 5/7 = 2/7

5. Nov 10, 2012

beaf123

Oh, I changed it now. It is correct so far, but I dont understand how they get

5/7ln(x-1) -2/7ln(x+6)

to become ( from solution):

(1/7)ln[(x+6)^2(x-1)^5] + C

6. Nov 10, 2012

SammyS

Staff Emeritus
That should be
5/7ln(x-1) + 2/7ln(x+6)​
Then use properties of logarithms to get the desired result.

Of course the given answer includes the constant of integration.