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17Calculus Differential Equations - Exponential Growth and Decay

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Although this is a differential equation topic, many students come across this topic while studying basic integrals. You do not need to know anything other than integrals to understand where the equations come from. If you are given the equation and not expected to derive it, you need only logarithms and algebra to work many problems.

quick notes

rate of change is proportional to quantity     \( y' = ky ~~~ \to ~~~ y = Ae^{kt} \)

You will definitely need to be sharp with your logarithms for this topic. The precalculus logarithms page will help you get up to speed.

The population dynamics page expands on this discussion of exponential growth and decay applied specifically to population change.

What Does Exponential Growth/Decay Mean?

What does it mean for something to grow or decay exponentially? The idea is that the independent variable is found in the exponent rather than the base. For example, comparing \(f(t)=t^2\) and \(g(t)=2^t\), notice that \(t\) is in the exponent of the \(g(t)\), so \(g(t)\) is considered an example of exponential growth but \(f(t)\) is not (since \(t\) is not in the exponent).
The general form of an exponential growth equation is \(y = a(b^t)\) or \(y=a(1+r)^t\). These equations are the same when \(b=1+r\), so our discussion will center around \(y = a(b^t)\) and you can easily extend your understanding to the second equation if you need to.
When \(b > 1\), we call the equation an exponential growth equation.
When \(b < 1\), it is called exponential decay.
When \(b = 1\), we have \(y=a(1^t)=a\), which is just linear equation and it is not considered an exponential equation.
So it is easy to see that we have the same equation in both cases and the value of \(b\) tells us if the exponential is growing or decaying. Let's watch a quick video before we go on.

PatrickJMT - An Exponential Growth Problem [10min-42secs]

video by PatrickJMT

A Special Type of Exponential Growth/Decay

A specific type of exponential growth is when \(b=e^r\) and \(r\) is called the growth/decay rate. The equation comes from the idea that the rate of change is proportional to the quantity that currently exists. This produces the autonomous differential equation

autonomous equation

A special type of differential equation of the form \(y' = f(y)\) where the independent variable does not explicitly appear in the equation.

\( y' = ky \)

where \(k\) is a constant called the growth/decay constant/rate.
When \(k < 0\), we use the term exponential decay.
When \(k > 0\), we use the term exponential growth.
To solve this differential equation, there are several techniques available to us. We will use separation of variables. Although not explicitly written in this equation, the independent variable we usually use in these types of equations is t to represent time.

\(\begin{array}{rcl} y' & = & ky \\ \displaystyle{\frac{dy}{dt} } & = & ky \\ \displaystyle{\frac{dy}{y} } & = & k~dt \\ \displaystyle{\int{ \frac{dy}{y} }} & = & \displaystyle{\int{ k~dt }} \\ \ln|y| & = & kt+C \\ e^{\ln|y|} & = & e^{kt+C} \\ y & = & Ae^{kt} \end{array}\)

The constant A is usually called the initial amount since at time \(t=0\), we have \(y = Ae^0 = A\).

The key to solving these types of problems usually involves determining \(k\). Depending on what your instructor wants, you can usually just start with the equation \(y = Ae^{kt}\), if you know that you have an exponential decay or growth problem.

Here is a quick video explaining this and showing a graph to give you a feel for these equations.

Brightstorm - The Differential Equation Model for Exponential Growth [2min-20secs]

video by Brightstorm

Half-Life

Half-life is the time it takes for half the substance to decay. The idea is to take the equation \(y = Ae^{kt}\), set the left side to \(A/2\) and solve for t. Notice that you don't have to know the initial amount A since in the equation \(A/2 = Ae^{kt}\), the A cancels leaving \(1/2 = e^{kt}\). You can then use basic logarithms to solve for t.

Uniqueness of Solution

So it may not have occurred to you but have you thought that maybe it is possible there is another solution to the differential equation \(x'=ax\)? This question is part of a very deep discussion in differential equations involving existence and uniqueness. Here is a really good video that shows that the solution discussed on this page is unique.

Dr Chris Tisdell - Why do initial value problems for x' = ax have exactly one solution? [6min-10secs]

video by Dr Chris Tisdell

Practice

You CAN Ace Differential Equations

Topics You Need To Understand For This Page

precalculus - logarithms

 

these topics are not required for this page but will help you understand where the equations come from

integration

basics of differential equations

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