Hey Guys, I learning about Related rates and although I understand the basic concepts, I'm stuggling with this problem - When two resistors r1 and R2 are connected in parallel, the total resistance R is given by the equation 1/R=1/R1+1/R2. If R1 and R2 are increasing at rates of .01 ohm/sec and .02 ohm/sec respectively, at what rate is R changing at the instant R1=30 ohms and R2= 90 ohms? My Work: dr1/dt= .01 dr2/dt= .02 dr/dt= Trying to find this when r1=30 and r2=90 First what I did was plug in R1 and R2 to get total resistance which i got to be 1/22.5. Next I took the derivative of that equation: -1/r1²(dr/dt)=-1/r²(dr1/dt)-1/r²2(dr/dt). I plugged in R1 and R2 along with dr1/dt and dr2/dt along with the total resistance and tried to solve for dr/dt (total resistance change). But I end up with .018 ohms/sec whereas the answer the book shows is 11/1600 or .006 ohm/sec. Where did i go wrong? Could someone please show me a easy to follow step by step in getting the answer ^_^, i would greatly appreciate it!.