• May 1st 2013, 02:57 AM
calmo11
From the equation $\frac 1 {R} = \frac 1 {R_1} + \frac 1 {R_2}$ you can rearrange to make 'R' the subject:
$R = \frac {R_1R_2}{R_1 + R_2}$
Now it's simply a matter of taking the derivative of this with respect to time, and substituting the values for $R_1, \ R_2, \ R'_1$ and $R'_2$ that you've been given.
Or use "implicit differentiation" as it stands. Do you know that $dx^{-1}/dt= -x^{-2}dx/dt$?