8. After x weeks the number of persons using a new drug was 
. At what percentage rate was the use of the drug changing after four weeks?
Solution: Let’s compute
We can take two approaches: by derivatives (approximation) or exactly:
After four weeks the derivative is computed as
which means that approximately 208 new people a week use the drug after four weeks. We also know that
the percentage rate the use of the drug is changing after four weeks is 10.4% (approximation using derivatives).
We can compute the percentage exactly as:
(b) By how much did the number of persons using this drug actually change during the fourth week?
Solution: We can find the exact change computing
