# Math Help - Help please

1. ## Help please

Any help with this would be wonderful

2. ## Re: Help please

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.

3. ## Re: Help please

Or use "implicit differentiation" as it stands. Do you know that $dx^{-1}/dt= -x^{-2}dx/dt$?