# Show that y(x,t)=y1(x,t)+y2(x,t) is a solution to the wave equation

## Homework Statement

Let y$_{1}$(x,t)=Acos(k$_{1}$x-ω$_{1}$t) and y$_{2}$(x,t)=Acos(k$_{2}$-ω$_{2}$t) be two solutions to the wave equation
$\frac{∂^{2}y}{∂x^{2}}$=$\frac{1}{v^{2}}$$\frac{∂^{2}y}{∂t^{2}}$
for the same v. Show that y(x,t)=y$_{1}$(x,t)+y$_{2}$(x,t) is also a solution to the wave equation.

## Homework Equations

Equations given in problem statement and

cos(a-b)=cosacosb+sinasinb

## The Attempt at a Solution

I am pretty sure I put y$_{1}$(x,t)+y$_{2}$(x,t) in a form that does not involve addition like 2Acosasinb or something close to that. Then I take a second partial derivative with respect to both x and t and show that the ratio of ∂$^{2}$y/∂t$^{2}$ over ∂$^{2}$2/∂x$^{2}$ is equal to v$^{2}$.
I am hung up on the trig.

y(x,t)=y$_{1}$(x,t)+y$_{2}$(x,t)

y(x,t)=Acos(k$_{1}$x-ω$_{1}$t)+Acos(k$_{2}$-ω$_{2}$t)

y(x,t)=A(cosk$_{1}$xcosω$_{1}$t+sink$_{1}$xsinω$_{1}$t)+A(cosk$_{2}$xcosω$_{2}$t+sink$_{2}$xsinω$_{2}$t)

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vela
Staff Emeritus
Homework Helper
You're making it too complicated. Don't use any trig identities. You know that y1 and y2 are solutions. What does that mean mathematically?

That their sum should also be a solution by the principle of superposition?

vela
Staff Emeritus
Homework Helper
Well, that's what you're supposed to show.

When you assume $y_i$ is a solution, you can say that
$$\frac{\partial^2}{\partial x^2}y_i(x,y) = \frac{1}{v^2}\frac{\partial^2}{\partial t^2} y_i(x,t),$$ where $\frac{\omega_i}{k_i} = v$.

So now plug $y=y_1+y_2$ into the lefthand side of the wave equation. You get
$$\frac{\partial^2}{\partial x^2}y = \frac{\partial^2}{\partial x^2}(y_1+y_2)$$ which you can split into two terms because differentiation is a linear operation. What you want to do is keep manipulating the righthand side until you end up with
$$\frac{1}{v^2} \frac{\partial^2}{\partial t^2}y.$$

$\frac{\partial^2}{\partial x^2}y = \frac{\partial^2}{\partial x^2}(y_1+y_2)$
$\frac{\partial^2}{\partial x^2}y = \frac{\partial^2 y_1}{\partial x^2} + \frac{\partial^2 y_2}{\partial x^2}$
$\frac{\partial^2}{\partial x^2}y = \frac{1}{v^2}\frac{\partial^2 y_1}{\partial t^2}+\frac{1}{v^2}\frac{\partial^2 y_1}{\partial t^2}$
$\frac{\partial^2}{\partial x^2}y = \frac{1}{v^2}(\frac{\partial^2 y_1}{\partial t^2}+\frac{\partial^2 y_2}{\partial t^2})$
$\frac{\partial^2}{\partial x^2}y= \frac{1}{v^2}\frac{\partial^2}{\partial t^2}(y_1+y_2)$
$\frac{\partial^2}{\partial x^2}y = \frac{1}{v^2}\frac{\partial^2}{\partial t^2}y$

Is that all I have to show since I already know that y is a solution to the wave equation and that it is assumed that is obeys that principle of superposition? Thanks again for your time.