This is given by:
\[\frac{\partial^{2} D}{\partial x^{2}} = \frac{\partial}{\partial x} \left( \frac{\partial D}{\partial x} \right)\]
We know that $\frac{\partial D}{\partial x} = A k \cos(kx) \cos(\omega t)$, so we differentiate this again with respect to $x$ to
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