If two resistors with resistances R1 and R2 are connected in parallel, as in the figure below, then the total resistance R, measured in ohms (Ω), is given by

Question

1
R
=
1
R1
+
1
R2
.
If R1 and R2 are increasing at rates of 0.3 Ω/s and 0.2 Ω/s, respectively, how fast is R changing when R1 = 80 Ω and R2 = 90 Ω? (Round your answer to three decimal places.)

Answer 1

well, just do the math

1/R = 1/R1 + 1/R2
plug in R1 and R2 to find R

1/R^2 dR/dt = 1/R1^2 dR1/dt + 1/R2^2 dR2/dt

Now just plug in your numbers to find dR/dt