Exponential Decay
Application
Example 1 Continued
Step 3. Write a model for the exponential decay.
Remember that the general formula for an exponential function is 
a is the initial value, so a = 20.
b is the common ratio, so b = 1/2.
Therefore, the equation is 
Step 4. Determine the amount of radioactivity left after three weeks.
Since x represents the number of weeks, let x = 3.
