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lovelymultani
Group Title
The halflife of censium137 is 30 years. Suppose we have a 200 mg sample. Let P(t) be the mass remaining after t years.
a) find differentiation equation satisfied by p(t)
b) find p(t) and simplify
c) how much mass remains after 75 years?
d) after how many years is the madd reduced to 1 mg?
 2 years ago
 2 years ago
lovelymultani Group Title
The halflife of censium137 is 30 years. Suppose we have a 200 mg sample. Let P(t) be the mass remaining after t years. a) find differentiation equation satisfied by p(t) b) find p(t) and simplify c) how much mass remains after 75 years? d) after how many years is the madd reduced to 1 mg?
 2 years ago
 2 years ago

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satellite73 Group TitleBest ResponseYou've already chosen the best response.2
\[p(t)=200\left(\frac{1}{2}\right)^{\frac{t}{30}}\] is the quick and easy answer, but perhaps not the one your math teacher wants
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
differentiated would be p(t) = .5^t/30 ?
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
you are probably being asked for \[p(t)=p_0e^{kt}\] where \(p_0=200\) (the initial value) and you have to solve for \(k\) via \[e^{30k}=\frac{1}{2}\]
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
you get \[30k=\ln(.5)\] or \[k=\frac{\ln(.5)}{30}\]
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
not sure what it means by "simplify" maybe write \[k=\frac{\ln(.5)}{30}=.0231\] and so your function is \[p(t)=200e^{.0231t}\]
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
ok i believe i have confused youi lets go slow
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
we can't use calculators on our final, so I have no way of getting .0231t . Is there an alternative?
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
you are going to have a function that is an exponential function it is going to look like \[p(t)=p_0e^{kt}\] to answer your question, no, you cannot know what \(\ln(.5)\div 30\) is without a calculator
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
in fact you can't do the third or fourth part without a calculator either you can write down an expression to solve, but you can't solve it
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
allright i guess I cant solve it, maybe they jst want the formulas.
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
that is weird
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
Yes, the decay formula, I have that. Next i plugged in the intital amount. I am just confused about what to put in for kt and yes =( it sucks
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
then you get \(k=\frac{\ln(\frac{1}{2})}{30}\) and leave it at taht
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
ok lets make sure you understand it you know that the initial value is \(200\) so you know \(p_0=200\) what you start with
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
therefore you function will look like \[p(t)=200e^{kt}\] but you do not yet know \(k\), we have to solve for it
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
but you know that the half life is 30 years. i.e. you know if \(t=30\) you must have \(p(30)=100\) (since 100 is half of 200) so you can solve for \(k\) by replacing \(t\) by 30 and \(p(30)=100\)
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
you get a) \[200e^{30k}=100\]b)\[e^{30k}=\frac{1}{2}\] we could have gone right to the second equation
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
solve via \[30k=\ln(\frac{1}{2})\] \[k=\frac{\ln(\frac{1}{2})}{30}\]
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
now if you are not allowed to use a calculator, then that is the best you can do you can write \[p(t)=200e^{\frac{\ln(.5)}{30}t}\]
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
which is very strange to me, but if you say so then i believe you
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
That was a great walk through , thanks soo much. Just confused on one last thing. that entire p(t) is for b, correct. u also said b) e^30k = 1/2 up there
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
to answer the next question replact \(t\) by \(75\), that is, compute \[p(75)\]
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
yeaa i go to city college in new york and they arent allowing any calcs for calc or precal
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
the work was finding \(k\) to use in the function \(p(t)=p_0e^{kt}\) the \(p_0=200\) part is easy , the hard part is finding \(k\) once you have it, the "final answer" is \[p(t)=200e^{\frac{\ln(.5)}{30}t}\]
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
now i am going to guess that if you cannot use a calculator on the final, there is no way they will ask you part 3 or 4 on a final, because you literally cannot do it
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
allright so when differentiated p(t) = ln(.5)/30*t ? and they do ask but I am going to go through notes to see how it was solved
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
hold on
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
they are not asking for the derivative, they are asking for the "differential equation"
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
the answer to the first question is \[\frac{dP}{dt}=kP\]
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
or maybe your text uses \[\frac{dP}{dt}=kP\] since this is for decay it makes no real different, since \(k\) is negative
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
this is the same as saying the instantaneous rate of change is proportional to the current amount
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
the solution to \[\frac{dP}{dt}=kP\] is \[P(t)=P_0e^{kt}\] which is what we solved above
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
i guess i should have said in your case \[\frac{dP}{dt}=kP\] and \[P(0)=200\]
 2 years ago

satellite73 Group TitleBest ResponseYou've already chosen the best response.2
we cannot do 3 or 4 without a calculator, but i can show you how to do them if you like
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
wouldnt a be kt ? from the original decay equation?
 2 years ago

lovelymultani Group TitleBest ResponseYou've already chosen the best response.0
ok nvm i got it ! thanks :)
 2 years ago
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