Friday, 16 November 2018

calculus - why do I have to double an exponential growth and half an exponential decay function to find out how fast/slow the process is?



Let’s take the exponential growth function to be y=A0ekt and exponential decay function to be y=A0ekt .




I’m just told that to find out How fast the process is, you have to double the exponential growth function and Half-time the exponential decay function. Why is this so? I never seem to understand it. Is there a way to prove it? Or is my tutor right by just telling me the fact and nothing else?


Answer



Let's take the exponential growth function f(t)=A0ekt. It is simple to find A0 - this is just the value of f at time 0. But how can we find the value of k ?



Suppose f(t) has value y0 at time t0 and value 2y0 at time t1. Then



2y0=2A0ekt0=A0ekt1



2ekt0=ekt1




ln(2)+kt0=kt1



k=ln(2)t1t0



The time interval t1t0 during which f(t) doubles is called the "doubling time". Notice that it does not depend on when it is measured. Once you know the doubling time you can find the growth constant k.



You can do the same calculation with exponential decay except you measure the time taken for f(t) to halve, not double, which is called the "half life".


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