3 is:
$\frac{\partial^2 u}{\partial t^2} = c^2 \frac{\partial^2 u}{\partial x^2}$
We can start by assuming a solution of the form:
$u(x,t) = X(x)T(t)$
Substituting this into Equation 19.3, we get:
$X(x)T''(t) = c^2 X''(x)T(t)$
Dividing both sides by
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