Problem 9. Show how to solve the following differential equation. You will need to use separation of variables Explain all your steps. Utz +41 = Uxx 02020, t50 tro 1(0₁6) = u(c₁ts=0 4 (2₁0) = f(₂), 14₂ (2₂0) =g6x) 02x5c- Ans. (an andnt + basim dot ) Sin Dix n-o an = ²/² √o b(x) Dinnux dx (x₁) = 12 bn = = = √₁ glus pinnix cla dn = (1²7) ² +4

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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**Problem 9**

**Objective:**
Show how to solve the following differential equation. You will need to use separation of variables. Explain all your steps.

**Equation:**
\[ U_{tt} + 4U = U_{xx} \]

**Conditions:**
For \(0 \leq x \leq c\), \(t \geq 0\):
- \(U(0,t) = U(c,t) = 0\), \(t \geq 0\)
- \(U(x,0) = f(x)\), \(U_t(x,0) = g(x)\), \(0 \leq x \leq c\)

**Solution:**

\[ U(x,t) = \sum_{n=0}^{\infty} \left( a_n \cos \lambda_n t + b_n \sin \lambda_n t \right) \sin \left(\frac{n \pi}{c} x \right) \]

**Coefficients:**

\[ a_n = \frac{2}{c} \int_0^c f(x) \sin \left(\frac{n \pi}{c} x \right) dx \]

\[ b_n = \frac{1}{\lambda_n} \frac{2}{c} \int_0^c g(x) \sin \left(\frac{n \pi}{c} x \right) dx \]

**Eigenvalue:**

\[ \lambda_n = \left(\frac{n \pi}{c}\right)^2 + 4 \]

**Note:**
This solution method involves expressing the solution \( U(x,t) \) as a Fourier series and determining the coefficients \( a_n \) and \( b_n \) using integrals over the given boundary conditions. The eigenvalue \( \lambda_n \) is modified by the constant term 4, which comes from the equation \( U_{tt} + 4U = U_{xx} \).
Transcribed Image Text:**Problem 9** **Objective:** Show how to solve the following differential equation. You will need to use separation of variables. Explain all your steps. **Equation:** \[ U_{tt} + 4U = U_{xx} \] **Conditions:** For \(0 \leq x \leq c\), \(t \geq 0\): - \(U(0,t) = U(c,t) = 0\), \(t \geq 0\) - \(U(x,0) = f(x)\), \(U_t(x,0) = g(x)\), \(0 \leq x \leq c\) **Solution:** \[ U(x,t) = \sum_{n=0}^{\infty} \left( a_n \cos \lambda_n t + b_n \sin \lambda_n t \right) \sin \left(\frac{n \pi}{c} x \right) \] **Coefficients:** \[ a_n = \frac{2}{c} \int_0^c f(x) \sin \left(\frac{n \pi}{c} x \right) dx \] \[ b_n = \frac{1}{\lambda_n} \frac{2}{c} \int_0^c g(x) \sin \left(\frac{n \pi}{c} x \right) dx \] **Eigenvalue:** \[ \lambda_n = \left(\frac{n \pi}{c}\right)^2 + 4 \] **Note:** This solution method involves expressing the solution \( U(x,t) \) as a Fourier series and determining the coefficients \( a_n \) and \( b_n \) using integrals over the given boundary conditions. The eigenvalue \( \lambda_n \) is modified by the constant term 4, which comes from the equation \( U_{tt} + 4U = U_{xx} \).
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