Pre Calculus
posted by Josh on .
Exponential Growth and Decay
Find the halflife of a radioactive substance that decays by 5% in 9 years.

x = xi e^kt
.95 = e^9 k
ln .95 = 9k
k = .005699
.5 = e^.005699 t
ln .5 =  .005699 t
t = 121.6 years