Laplace Transform 9190A7
1. **State the problem:** Solve the initial value problem using the Laplace transform:
$$y'' + 3y' = 0, \quad y(0) = 6, \quad y'(0) = 4$$
2. **Apply the Laplace transform:** Let $Y(s) = \mathcal{L}\{y(t)\}$. Using linearity and the transforms of derivatives:
$$\mathcal{L}\{y''\} = s^2 Y(s) - s y(0) - y'(0) = s^2 Y(s) - 6s - 4$$
$$\mathcal{L}\{y'\} = s Y(s) - y(0) = s Y(s) - 6$$
3. **Transform the differential equation:**
$$s^2 Y(s) - 6s - 4 + 3(s Y(s) - 6) = 0$$
4. **Simplify the equation:**
$$s^2 Y(s) - 6s - 4 + 3s Y(s) - 18 = 0$$
Group terms with $Y(s)$ and constants:
$$(s^2 + 3s) Y(s) - 6s - 4 - 18 = 0$$
$$(s^2 + 3s) Y(s) = 6s + 22$$
5. **Solve for $Y(s)$:**
$$Y(s) = \frac{6s + 22}{s^2 + 3s} = \frac{6s + 22}{s(s + 3)}$$
6. **Partial fraction decomposition:**
Assume:
$$\frac{6s + 22}{s(s + 3)} = \frac{A}{s} + \frac{B}{s + 3}$$
Multiply both sides by $s(s+3)$:
$$6s + 22 = A(s + 3) + Bs$$
Set $s=0$:
$$22 = 3A \implies A = \frac{22}{3}$$
Set $s = -3$:
$$6(-3) + 22 = B(-3) \implies -18 + 22 = -3B \implies 4 = -3B \implies B = -\frac{4}{3}$$
7. **Rewrite $Y(s)$:**
$$Y(s) = \frac{22/3}{s} - \frac{4/3}{s + 3}$$
8. **Inverse Laplace transform:**
Recall:
$$\mathcal{L}^{-1}\left\{\frac{1}{s}\right\} = 1$$
$$\mathcal{L}^{-1}\left\{\frac{1}{s + a}\right\} = e^{-at}$$
So,
$$y(t) = \frac{22}{3} - \frac{4}{3} e^{-3t}$$
**Final answer:**
$$\boxed{y(t) = \frac{22}{3} - \frac{4}{3} e^{-3t}}$$