## Appendix: 1. Exponential Decay

As we already know, a drug will *decay exponentially*, meaning that it will decompose naturally. First, we consider the mathematics of exponential decay.

Let , be a function of time , defined as , for some positive . This equation governs the mathematics of exponential decay. We also need to define some *initial condition*, in other words, we need to define the value of , say .

Why is the equation called an exponential function?

If is positive, does it make sense that there is a negative sign in the exponent? Why?

Thus, in the Figure 1 below we see the exponential decay equation, , depicted graphically.

What occurs to the curve as more and more time passes?

In Exponential Decay: Part 2, we will explore the effect of on the exponential decay curve.