S∂u1/∂t +8∂u2/∂x =0 ∂u2/∂t +∂u1/∂x =0. u1(x,0)=sin. u2(x,0)=x²

You have provided a system of partial differential equations (PDEs) with initial conditions. Let's carefully analyze and solve this system step by step.

### Given system of PDEs:
\[
\frac{\partial u_1}{\partial t} + 8 \frac{\partial u_2}{\partial x} = 0
\]
\[
\frac{\partial u_2}{\partial t} + \frac{\partial u_1}{\partial x} = 0
\]

### Initial conditions:
\[
u_1(x, 0) = \sin(x)
\]
\[
u_2(x, 0) = x^2
\]

#### Step 1: Decouple the system
We will differentiate both equations with respect to \( t \) and \( x \), respectively, and try to eliminate one of the unknowns.

1. Differentiate the first equation with respect to \( t \):
\[
\frac{\partial^2 u_1}{\partial t^2} + 8 \frac{\partial^2 u_2}{\partial t \partial x} = 0
\]

2. Differentiate the second equation with respect to \( x \):
\[
\frac{\partial^2 u_2}{\partial t \partial x} + \frac{\partial^2 u_1}{\partial x^2} = 0
\]

Now, using the second equation to substitute \( \frac{\partial^2 u_2}{\partial t \partial x} = -\frac{\partial^2 u_1}{\partial x^2} \) into the first:
\[
\frac{\partial^2 u_1}{\partial t^2} - 8 \frac{\partial^2 u_1}{\partial x^2} = 0
\]

This is a standard **wave equation** for \( u_1 \):
\[
\frac{\partial^2 u_1}{\partial t^2} = 8 \frac{\partial^2 u_1}{\partial x^2}
\]
The general solution for this wave equation is:
\[
u_1(x, t) = f(x - \sqrt{8}t) + g(x + \sqrt{8}t)
\]
where \( f \) and \( g \) are arbitrary functions determined by the initial conditions.

#### Step 2: Solve for \( u_1(x, t) \)
Using the initial condition \( u_1(x, 0) = \sin(x) \), we get:
\[
f(x) + g(x) = \sin(x)
\]

Now, differentiate \( u_1(x, t) \) with respect to \( t \):
\[
\frac{\partial u_1}{\partial t} = -\sqrt{8} f'(x - \sqrt{8}t) + \sqrt{8} g'(x + \sqrt{8}t)
\]
Using the first PDE to substitute \( \frac{\partial u_1}{\partial t} \) into \( \frac{\partial u_2}{\partial x} \), we will find \( u_2(x,t) \).

Would you like to continue with the derivation of the full solution for both \( u_1 \) and \( u_2 \), or would you prefer a specific part explained further?

Here are five related questions you might consider:
1. How do we solve the wave equation for other types of initial conditions?
2. What are the properties of wave equations in different contexts?
3. How does the decoupling method work for more complex systems of PDEs?
4. Can we solve this system numerically instead of analytically?
5. How do the initial conditions impact the form of the solution?

**Tip**: For wave equations, understanding the relationship between characteristic lines and wave propagation helps interpret the physical meaning of solutions.

Learn how to solve a system of partial differential equations involving a wave equation with initial conditions u1(x, 0) = sin(x) and u2(x, 0) = x². This solution walks through the decoupling method and wave equation theory, suitable for university-level mathematics.

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