Partial Fractions Integration 2B7D26
1. **State the problem:** Express the rational function $$\frac{19 - x}{(x - 7)(x + 4)}$$ as the sum of its partial fractions and then find the integral $$\int \frac{19 - x}{(x - 7)(x + 4)} \, dx$$.
2. **Set up the partial fractions:** We want to write
$$\frac{19 - x}{(x - 7)(x + 4)} = \frac{A}{x - 7} + \frac{B}{x + 4}$$
where $$A$$ and $$B$$ are constants to be determined.
3. **Multiply both sides by the denominator:**
$$19 - x = A(x + 4) + B(x - 7)$$
4. **Expand the right side:**
$$19 - x = A x + 4A + B x - 7B = (A + B) x + (4A - 7B)$$
5. **Equate coefficients:**
For the $$x$$ terms:
$$-1 = A + B$$
For the constant terms:
$$19 = 4A - 7B$$
6. **Solve the system:**
From $$-1 = A + B$$, we get $$A = -1 - B$$.
Substitute into the second equation:
$$19 = 4(-1 - B) - 7B = -4 - 4B - 7B = -4 - 11B$$
Add 4 to both sides:
$$23 = -11B$$
So,
$$B = -\frac{23}{11}$$
Then,
$$A = -1 - \left(-\frac{23}{11}\right) = -1 + \frac{23}{11} = \frac{12}{11}$$
7. **Rewrite the partial fractions:**
$$\frac{19 - x}{(x - 7)(x + 4)} = \frac{12/11}{x - 7} - \frac{23/11}{x + 4}$$
8. **Integrate term-by-term:**
$$\int \frac{19 - x}{(x - 7)(x + 4)} \, dx = \int \frac{12/11}{x - 7} \, dx - \int \frac{23/11}{x + 4} \, dx$$
9. **Use the integral formula:**
$$\int \frac{1}{x - a} \, dx = \ln|x - a| + C$$
10. **Final integral:**
$$= \frac{12}{11} \ln|x - 7| - \frac{23}{11} \ln|x + 4| + C$$
**Answer:**
$$A = \frac{12}{11}, \quad B = -\frac{23}{11}$$
$$\int \frac{19 - x}{(x - 7)(x + 4)} \, dx = \frac{12}{11} \ln|x - 7| - \frac{23}{11} \ln|x + 4| + C$$