# Homework Help: Help with partial fractions

1. Aug 9, 2010

### TsAmE

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

Evaluate the integral:

$$\int_{1}^2 \frac{4y^2 - 7y -12}{y(y+2)(y-3)}dy$$

2. Relevant equations

None.

3. The attempt at a solution

$$\frac{4y^2 -7y -12}{y(y+2)(y-3)} = \frac{A}{y} + \frac{B}{y+2} + \frac{D}{y-3}$$

$$y = 0: -12 = -6A \rightarrow A=2$$

$$y = -2: 16 + 14 - 12 = 10B$$

$$18 = 10B \rightarrow B = \frac{18}{10} = \frac{9}{5}$$

$$y = 3: 36 - 21 -12 = 15D$$

$$3 = 15D \rightarrow D = \frac{3}{15} = \frac{1}{5}$$

=$$\int_{1}^2 \frac{2}{y} + \frac {\frac{9}{5}}{y + 2} + \frac{\frac{1}{5}}{y - 3} dy$$

=$$2ln|y| + 9ln|5y+2| + ln|y - 3| ]_{1}^{2}$$

$$2ln2 + 9ln12 - (9ln7 + ln2)$$

$$ln2 + 9ln12 - 9ln7$$

The correct answer is $$\frac {27}{5}ln2 - \frac{9}{5}ln3$$ but I cant see what I did wrong.

2. Aug 9, 2010

### Staff: Mentor

The mistakes are above. The 2nd and 3rd terms in your antiderivative are incorrect. I was able to get the same answer as in the book.