Man, a question about Math, and I am late! I agree with Paul about an ODE. They are commonly used to model many, many, many things, especially in the human body -- Here is the solution of a linear, homogeneous ordinary differential equation, solved by seperation of variables (this is pretty much the easiest differential equation there is).
dx/dt = x given x(0) = 0
S(0 --> x) dx/x = S(0 --> t) dt
ln(x)-ln(0) = t-0
(ln(x) - 1) = t
e^ln(x) = e^(t+1)
x = e^(t+1)
In words, d x by d t equals x with an initial condition of x = 0 at t - 0. You solve by seperation of variables. First, you split up the equation getting all terms with t on the rigth side and all terms with x on the left. Then, you take the integral of both sides, from 0 to x on the left and 0 to t on the right, using your initial condition. The integral on the left, with respect to x comes out as a natural log while the integral on the right with respect to t just comes out as t. Then you plug in your limits of integration, and you can solve by taking the exponential of both sides to get rid of the ln term.
QED
