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Q: The voltage V in volts across a fixed resistance r in series with a variable resistance R is V=(rE)/(R+r) where E is the source voltage. calculate the rate at which V changes with respect to time if when E=10 volts, R=12 ohms, and r=8 ohms, the source voltage is increasing at 2 volts per minute and the variable resistance is decreasing at two ohms per minute.