Partial Fractions Integration 92F79C
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. **Partial fraction decomposition formula:** For distinct linear factors, we 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 of like terms:**
- Coefficient of $x$: $-1 = A + B$
- Constant term: $19 = 4A - 7B$
6. **Solve the system:**
From $-1 = A + B$, we get $A = -1 - B$.
Substitute into $19 = 4A - 7B$:
$$19 = 4(-1 - B) - 7B = -4 - 4B - 7B = -4 - 11B$$
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. **Evaluate the integrals:**
$$= \frac{12}{11} \ln|x - 7| - \frac{23}{11} \ln|x + 4| + C$$
**Final answer:**
$$\int \frac{19 - x}{(x - 7)(x + 4)} \, dx = \frac{12}{11} \ln|x - 7| - \frac{23}{11} \ln|x + 4| + C$$