Math
posted by Joy .
After 150 thousand years, only 1/8 of the original amount of a particular radioactive waste will remain. The halflife of this radioactive waste is how many thousand years?

Math 
Eren Jaeger
N = (No)e^(kt)
where
No is the original amount
k is constant
t is time
substitute,
(1/8)*No = No * e^(k*150000)
the No cancels:
1/8 = e^(150000k)
ln (1/8) = 150000k
k = 150000/ ln(1/8)
after solving for k, substitute to this equation and get the value of t (halflife):
1/2 * No = No * e^(kt) 
Math 
Joy
What does the math symbols such as ^ and * mean? I still don't understand exactly what your trying to solve for. Am I trying to figure out the halflife of radioactive waste? Am I trying to figure out how many thousand years it takes in a halflife of radioactive waste? What does the term in math mean when saying "halflife"? Is the 1/8 suppose to stay as a fraction or be turned into a decimal or percent?

Math 
Eren Jaeger
the symbol ^ means "raise to". the symbol * means "multiplied by" as in for example
3^2 = 3*3 = 9
Yes, we're looking for the halflife. Halflife is the time it takes for a substance to achieve 1/2 of its original amount when it undergoes radioactive decay. the formula used for radioactive decay is
N = (No)*e^(k*t)
where
N = remaining amount after time, t
No = the original amount
k = some constant
t = time
since we don't know the k value (nothing is given), as well as the original amount (the No), express N in terms of No so we can cancel them on both sides. that's why the equation becomes
(1/8)*No = No * e^(k*150000)
It is stated in the problem that after 150000 years (this is the variable t), the remaining amount will only be 1/8 of the original (the original is No, right?).
since the equation above contains No term, we can cancel them:
(1/8) = e^(k*150000)
then get the natural log of both sides. That will cancel the e on the right side, leaving only the exponent:
ln (1/8) = ln (e^(k*150000))
ln (1/8) = 150000k
k = 150000/ ln(1/8)
Get a calculator and solve the above equation to get k.
Now, after getting k, you need to substitute it to the original equation:
N = (No)*e^(k*t)
The N here will be equal to (1/2)*No since halflife (remember the definition?). Again, No will be cancelled on both sides. The equation becomes:
1/2 * No = No * e^(kt)
1/2 = e^(k*t)
ln (1/2) = ln(e^(k*t))
ln (1/2) = k*t
k = (ln(1/2)) / t
now you know the value of k, solve for t. 
Math 
Eren Jaeger
i mean,
t = (ln(1/2)) / k
we're solving for t. the units are in years. 
Math 
Steve
Wow  that's a lot of work.
each halflife diminishes the amount by half.
1/8 = 1/2 * 1/2 * 1/2
So, 150,000 years is 3 halflives
The halflife is thus 50,000 years.
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