Integral Partial Fractions
1. **Stating the problem:** We need to compute the integral
$$\int \frac{16x^2}{(x-18)(x+6)^2} \, dx.$$
2. **Set up partial fractions:** Because the denominator factors into a linear term and a squared linear term, express as
$$\frac{16x^2}{(x-18)(x+6)^2} = \frac{A}{x-18} + \frac{B}{x+6} + \frac{C}{(x+6)^2}.$$
3. **Multiply both sides by the denominator:**
$$16x^2 = A(x+6)^2 + B(x-18)(x+6) + C(x-18).$$
4. **Expand terms:**
$$A(x^2 + 12x + 36) + B(x^2 - 12x - 108) + C(x - 18).$$
5. **Group by powers:**
$$x^2: A + B$$
$$x: 12A - 12B + C$$
$$constant: 36A - 108B - 18C$$
6. **Match coefficients with left side $16x^2 + 0x + 0$:**
$$A + B = 16$$
$$12A - 12B + C = 0$$
$$36A - 108B -18C = 0.$$
7. **Solve system:**
From the first, $B = 16 - A$.
Substitute into second:
$$12A - 12(16 - A) + C = 0 \Rightarrow 12A - 192 + 12A + C = 0 \Rightarrow 24A + C = 192,$$
so $C = 192 - 24A$.
Substitute $B$ and $C$ into third:
$$36A - 108(16 - A) - 18(192 - 24A) = 0,$$
$$36A - 1728 + 108A - 3456 + 432A = 0,$$
sum coefficients for $A$:
$$36A + 108A + 432A = 576A,$$
constants sum:
$$-1728 - 3456 = -5184,$$
so
$$576A - 5184 = 0 \Rightarrow 576A = 5184 \Rightarrow A = 9.$$
Then
$$B = 16 - 9 = 7,$$
$$C = 192 - 24(9) = 192 - 216 = -24.$$
8. **Rewrite the integral:**
$$\int \frac{9}{x-18} + \frac{7}{x+6} - \frac{24}{(x+6)^2} \, dx = \int \frac{9}{x-18} \, dx + \int \frac{7}{x+6} \, dx - \int \frac{24}{(x+6)^2} \, dx.$$
9. **Integrate term-by-term:**
$$9 \ln|x-18| + 7 \ln|x+6| + 24 \frac{1}{x+6} + C,$$
note that integral of $-\frac{24}{(x+6)^2}$ is $24/(x+6)$ because:
$$\int \frac{1}{(x+a)^2} dx = -\frac{1}{x+a} + C.$$
10. **Final answer:**
$$\int \frac{16x^2}{(x-18)(x+6)^2} \, dx = 9 \ln|x-18| + 7 \ln|x+6| + \frac{24}{x+6} + C.$$