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- May 1st 2013, 02:57 AM #1

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- May 1st 2013, 06:17 AM #2
## Re: Help please

From the equation $\displaystyle \frac 1 {R} = \frac 1 {R_1} + \frac 1 {R_2}$ you can rearrange to make 'R' the subject:

$\displaystyle R = \frac {R_1R_2}{R_1 + R_2} $

Now it's simply a matter of taking the derivative of this with respect to time, and substituting the values for $\displaystyle R_1, \ R_2, \ R'_1$ and $\displaystyle R'_2$ that you've been given.

- May 1st 2013, 06:29 AM #3

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