Two variable resistors R1 and R2 are connected in parallel so that their combined resistance R is given by

1/R = 1/R1 + 1/R2

At an instant when R1 = 250 ohms and R2 = 1000 ohms, R1 is increatsing at a rate 100 ohms/minute. How fast must R2 be changing at that moment (a) to keep R constant? and (b) to enable R to increase at a rate of 10 ohms/minute?

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Hello!

I have been sitting with the above exercise for a while now but cannot figure how to solve it. I assume I need to solve the derivative of R (or 1/R) for when it is 0, but cannot evet get that far. I would appreciate any help I can get.

Thanks!