The Quotient Rule

A water tank fills and drains simultaneously, causing the water level to change due to unequal inflow and outflow rates. The goal is to find out how long it takes for the tank to drain. This is expressed as the ratio of water volume to drainage rate, which changes over time as both are differentiable functions of time.

As water enters the tank, the total volume increases slightly, while the drainage rate also shifts due to system adjustments. These small changes influence the overall volume-to-drainage quotient.

To find the quotient's rate of change, consider the change in the quotient over a small time interval, and then divide this change by that interval.

By introducing a limit as the time interval approaches zero, delta R also approaches zero because R varies continuously with time, simplifying the denominator. Using the definition of derivatives, the quotient rule is derived.

It equals the drainage rate times the derivative of the volume, minus the volume times the derivative of the drainage rate, all divided by the square of the drainage rate.

This final expression shows how the ratio of the water volume to the drainage rate changes over time. In general, this expression also relates how the ratio of two arbitrary functions changes over time.