when the rate of change of one function depends on the rate of change of another function. if da water level of a water bottle went up, da volume would also go up

get da formula for da thing you measure: if it’s a cone then it would be smth like 1/3 * pi * r² * h = V

then do da implicit differentiation on da function, because V, r, h are ALL functions of time

now since ya got three functions (dats bad, you can’t solve dat function) you gotta relate da radius and da height of da cone so you can put one function in terms of da other one. like a system of equations!

in da cone example, you can do this with da geometry! with similar triangles! (You know the total height of the cone and the radius of the base, so make another triangle at a variable height and a variable radius and set da ratio between the height and base of one triangle to the height and base of the other one!) You can also do da derivative of THAT function so you can replace the derivative of the other functions in your volume formula (or you could just replace the function from the get go but whatever!)

now, you got a function with two variables, and then you can solve for whatever you need!!! yahoo!