at what rate is R changing

Question

The combined electrical resistance R of two resistors R1 and R2, connected in parallel, is given by the equation below, where R, R1, and R2 are measured in ohms. R1 and R2 are increasing at rates of 1.0 and 2.0 ohms per second, respectively.

1/R =1/R1 + 1/R2

At what rate is R changing when R1 = 35 ohms and R2 = 80 ohms? (Round your answer to three decimal places.)

ohm/sec

Answer 1

The only thing you need to know here is that if f(x)=(1/x), then f'(x) = -1/x²

^{of course, the derivative of a sum is the sum of the derivatives.}

^{Be careful about the signs!}