Homework Help: Pre Calculus

Posted by Josh on Saturday, October 22, 2011 at 7:23pm.

Exponential Growth and Decay

Find the half-life of a radioactive substance that decays by 5% in 9 years.

Pre Calculus - Damon, Saturday, October 22, 2011 at 7:29pm
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

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