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## Homework Statement

Take ∂

^{2}E/∂t

^{2}E(r,t)=E

_{0}cos((k(u^·r−ct)+φ) in which u^ is a unit vector.

## Homework Equations

d/dx(cosx)=-sinx

## The Attempt at a Solution

I had calc 3 four years ago and can't for the life of me remember how to differentiate the unit vector. I came up with -c

^{2}u

^{2}^cos(k(u^r-ct)+φ), but I feel like I'm not doing that right.

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