# Need help differentiating this equations

I would like to know how differentiate this equation it should be fairly simple but its been a long night and I cant seem to figure it out.

$r =\frac{f}{R . \hat z} R$

with respect to time.

• f is some constant (focal length)
• R is a position vector
• $R . \hat z$ is inner product

Any help will be appreciated

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Did you mean $\frac{f}{R.z\hat R}$? You get this as $\frac{f}{R.z\hat R}$. – Martin Sleziak Oct 24 '12 at 18:10
Please edit your question to make it more readable. – Hagen von Eitzen Oct 24 '12 at 18:10
I'm guessing it should be $R (\hat z \cdot R)$. – Javier Oct 24 '12 at 18:12
sorry about that, it has now been correctly formatted – Alex Oct 24 '12 at 18:13
Does $\hat{z}$ depends on time? – Pragabhava Oct 24 '12 at 18:15

Assuming the direction of $\hat z$ does not change with time, \begin{align} \dot r &= \frac{\mathrm{d}}{\mathrm{d}t}\left(\frac{fR}{\langle R, \hat z\rangle}\right)\\ & = \frac{f}{\langle R, \hat z\rangle^2}\left[\langle R, \hat z\rangle \dot R- R\frac{\mathrm{d}}{\mathrm{d}t}\left(\langle R, \hat z\rangle\right)\right]\\ & = \frac{f}{\langle R, \hat z\rangle^2}\left[\langle R, \hat z\rangle \dot R- \langle \dot R, \hat z\rangle R\right] \end{align}