Drawing Decay

Decay uses the same rule, $y = a\cdot {b}^{x}$, with b between 0 and 1. Each step multiplies by a number less than 1. So the amount gets smaller, and the curve goes down.

A medicine in the body halves every 6 hours. We say its half-life is 6 hours. From 800 mg it goes to 400, then 200, then 100. Half of an amount is never zero. So the curve gets closer to the x-axis but never touches it.