# Applications for logarithms in a business math course

Context: I am an assistant professor at a small liberal arts institution in the US.

The title really says it all: I am looking to provide applications for logarithms in a business math course (most students are freshman). The sciences have a ton of uses for logs, but I'd like to have something specific to business/accounting/finance/economics.

Of course you can use them to solve questions related to exponential growth/decay problems, but I'd like to find examples where a logarithmic function itself describes the behavior we're examining. I've looked in a few business math textbooks and haven't come up anything terribly compelling thus far.

Examples I've written myself basically all hinge on some sort of "diminishing return," like the price of a tech stock immediately after its IPO, the effects of continued spending on advertising, or the depreciation of electronic equipment.

I'm a mathematician (not a businessperson), so I am suspicious there might be some ideas I'm missing, especially in economics. Any suggestions or advice is highly appreciated!

• Is "doubling time" for an investment (at constant interest) too obvious an application for you? Aug 18 at 1:23
• Is this for high school or some other level?
– Wyck
Aug 18 at 16:00
• To respond to the comments: 1. I currently use doubling time, but I'd like to find something where the "log part" of the problem is not based on some preexisting exponential behavior. 2. This is for college freshman, mostly. I've edited the question to clarify as well. Aug 18 at 17:24
• I'd expect that most applications of logarithms are to solve "when?" questions about exponential growth. For instance Declining balance depreciation, Example 7.7.8. Aug 18 at 18:23

## 7 Answers

### Log scales can introduce logarithms swimmingly! Let's start with a germane xkcd comic

Thanks to user Nat for charting the log-10 version in the comments! But I matched Nat's colors to mine.


• I edited out the offensive word. (When quoting reddit, it's your responsibility to make sure the text follows our code of conduct.) I see that this post has already been viewed 1000 times. I will remove offensive language more quickly in future. Aug 19 at 20:09
• No problem. I edited again. The math answer does not need to refer to the original language and what the problem with it is, so I took out that link. If people follow the first reddit link for this, then they're in reddit land, which I don't moderate. All set. Aug 20 at 19:16
• @Nat somebody is curious how you created the chart. See matheducators.meta.stackexchange.com/questions/736/…
– quid
Aug 26 at 10:11

I feel compelled to provide your "of course" answer of exponential growth/decay. This answer is hopefully appropriate for lower-level business courses such as high-school level.

Here's a totally reasonable, practical, and compelling question where logarithms are used to get the answer.

### How many times does something need to change by a percentage?

I was training to run a half-marathon (22 km). The advice given to me was:

Do not increase your running distance by more than 10% each week.

So, if I was currently running 5 km, I would need to know how many times I needed to increase my distance by 10% in order to reach my 22 km goal. I was trying to set my expectations as to how many weeks it would take me.

Each week my distance should be 1.10 times what it was the previous week.

This makes me ask $$5 \times 1.1^n = 22$$

Without logarithms you can only get as far as $$1.1^n=22/5$$

If you're unfamiliar with exponents and logarithms, it can be intimidating to see your variable trapped in the exponent like that and you need a more advanced mathematical technique that can get it out. Now with logarithms you can do $$log(1.1^n) = log(22/5)$$

And knowing a little about logarithmic identities you can free the variable from its exponential prison $$n \times log(1.1) = log(22/5)$$ and finally $$n = {log(22/5)\over log(1.1)} \approx 15.5$$

The advice is for me not to try to do this in fewer than about 16 weeks of training.

If you take this same problem and just change the story to suppose that I invest \$5,000 and I can increase my money by 10% per year, and ask "How many years will it take until I have \$22,000?" then it's a business application related to planning and computing the answer in variations of the scenario builds intuition about exponential growth/decay. (The answer is the same: 16 years.)

It's an "aha" moment to do this and realize that maybe it's not practical to wait 16 years for my \$5,000 investment to grow to the \$22,000 I need to buy a car that I need soon, for example. But a parent could invest \$5,000 at 10% when a child is born in order to afford that same \$22,000 car when the child turns 16 (a common legal driving age).

Compound interest and cumulative compounding are the classic financial examples.

A search on "investment math" will give you several books, problems, sites. I would suggest to buy one or check out a few in a college library. Below is a link to a pdf of one by Hart:

https://archive.org/details/mathematicsofinv004275mbp/page/n15/mode/2up

If you want a shorter, simpler take, than see the single chapter on investment math that Hart included in his college algebra text (war manual EM 315). You can get it from a library (use ILL if needed) or just buy it for a few bucks on a used book website.

Find some processes that are naturally exponential (investments, chemical processes, PSA test values in Prostate cancer patients). Get some log graph paper. Get them to graph this data on both normal graph paper and on log graph paper.

If you can find a lot of examples from radically different knowledge areas, it will be more effective.

Get a slide rule (yes, you can still find them online (there used to be an online shop in Vancouver Canada, but it looks like it's run out - you can find them on Etsy and eBay)). There are slide rule apps for phones.

Show the class how people multiplied and divided large numbers before the advent of calculators.

As an aside:
I have had prostate cancer. After my prostate was removed, my PSA count should have dropped to zero, but it's not unusual to see it greater than zero for several months. Typical PSA counts for folks my age are 2-3 ng/ml. With no prostate, I get "ultra-sensitive" PSA tests, that measure a few orders of magnitude lower than that. It takes a while before testing starts again after a prostatectomy. Mine were definitely non-zero. After 4 tests, the doctors kept saying "don't worry, it's so low". But, those 4 tests, when graphed (taking into account some measurement noise) formed a near perfect exponential curve, with a doubling time of about 4-5 months. Don't worry, radiation got rid of the troubling cancer cells

A fundamental concept in economics is elasticity, which is closely related to logarithms:

Consider two economical quantities $$x,y \in (0,\infty)$$, where $$y$$ depends on $$x$$. The elasticity $$\epsilon$$ of $$y$$ with respect to $$x$$ is defined as $$\epsilon(x) := \frac{\partial y}{\partial x} \,\frac{x}{y}.$$ Here are two ways to see a relation to logarithms:

• Due to the chain rule, one has $$\epsilon(x) = \frac{\partial \log(y)}{\partial x} x$$. This is, at least at first glance, purely computational; a more conceptual perspective is as follows:

• Let us consider a "logarithmic transformation of variables", i.e., we define new quantities $$u := \log(x)$$ and $$v := \log(y)$$ (which are thus elements of $$\mathbb{R}$$ rather than $$(0, \infty)$$). Then, again by the chain rule, $$\frac{\partial v}{\partial u} = \frac{\partial y}{\partial x} \frac{x}{y} = \epsilon.$$ In other words, the elasticity of $$y$$ with respect to $$x$$ is simply the derivative in logarithmic variables.

A Cobb-Douglas production/utility function has the form $$U(x,y,z,\dots)=K\,x^ay^bz^c\cdots \tag{1}$$ where $$U$$ represents the utility or level of production for given levels $$x,y,z,\dots$$ of "inputs" like capital, labor, materials, energy, and so forth. A typical modeling application is to maximize $$U$$ subject to constraints on the inputs. The typical constraint is to measure the inputs in terms of dollars and require the total to be within budget. For such a problem one can set $$K=1$$, if you're a mathematician; others might insist on proper units, so that they can relate solutions to what they know about their factories and sales.

What is the significance of the exponents $$a,b,c,\dots$$?

We have (ignore $$K$$ if you wish): $$\log U = a \log x + b \log y + c \log z + \log K \tag{2}$$ The differential of $$\log w$$ is the relative change $$dw/w$$, which the students should think of in the common phrase, "percent growth." They should think of "percent growth" as the derivative of the logarithm of the quantity. In our case, we obtain $${dU \over U} = a\,{dx \over x} + b\,{dy \over y} + c\,{dz \over z} \tag{3}$$ We see that the exponents appear as the coefficients in a linear combination of the percent changes in the inputs. They indicate the relative weights of the percent changes in the inputs. Together, this linear combination models the percent change in utility. At maximum utility, $$dU=0$$ and the linear combination has to "balance." An increase in $$x$$ ($$dx>0$$) is met by an overall decrease in the remaining terms. In terms of a budget constraint, if you spend more on $$x$$, you have less to spend on $$y$$ and $$z$$. It turns out that at maximum utility, one spends on $$x,y,z$$ amounts proportional to $$a:b:c$$.

The role of logarithms here, algebraically, is rather minor. As a person who attended a small liberal arts institution in the US, what I liked about my math program was an emphasis on concept, in particular, on applying concepts in thinking about problems. There are a couple that stand out to me. One I have already emphasized, the connection between logarithms and relative change. The other is how logarithms convert an exponent form like (1) into linear forms (2) and (3), and linear is much easier to understand. Of course one can proceed (3) $$\rightarrow$$ (2) $$\rightarrow$$ (1) via the exponential function since it is the inverse of the logarithm, another important concept. These concepts are mathematical, not business concepts. One builds on business concepts to create a mathematical model. Then one applies whatever mathematics is appropriate.

• How useful are Cobb-Douglas production functions in the real world (not counting their appearance in economics courses)?
– KCd
Aug 19 at 22:05
• @KCd Idk, but surely less useful than calculus in the real world (in business/econ outside of academia, if that's your real work). Here's one assessment: cbo.gov/sites/default/files/cbofiles/ftpdocs/94xx/doc9497/… - Here's a study of it in a 2012 thesis in Construction Engineering and Management, digitalcommons.unl.edu/constructiondiss/13 - just some random hits, though. "Not dead yet" I guess? Aug 19 at 22:35

Growth rates, especially of macroeconomic variables, that are usually small in value, are routinely modeled theoretically and estimated empirically by using logarithms. Let $$Y_t$$ be, say, gross domestic product, or national income, or aggregate consumption, at period $$t$$, (which is usually a year or a quarter), then its growth rate compared to period $$t-1$$ is

$${\rm growth\; rate} \equiv \frac{Y_{t} - Y_{t-1}}{Y_{t-1}} = \frac{\Delta Y_{t}}{Y_{t-1}}.$$

Consider the natural logarithm of the ratio of $$Y_t$$ over $$Y_{t-1}$$, while writing $$Y_t \equiv Y_{t-1} + \Delta Y_t$$,

$$\ln \left(\frac{Y_{t}}{Y_{t-1}}\right) = \ln \left(\frac{Y_{t-1} + \Delta Y_t}{Y_{t-1}}\right) = \ln \left(1+ \frac{\Delta Y_t}{Y_{t-1}}\right) \approx \frac{\Delta Y_t}{Y_{t-1}} = {\rm growth\;rate}$$

This approximation is acceptable when $$|\Delta Y_t/Y_{t-1}| < 0.1$$, which is almost always the case with macroeconomic variables in quarterly frequency, or even in yearly frequency. By the properties of the logarithms then,

$$\ln Y_{t} - \ln Y_{t-1} \approx {\rm growth\;rate\; of}\; Y_t.$$

For micro-level, business level though, this may not be advisable to use, since single business may experience growth (or contraction) rates higher than $$10\%$$ in their revenues, profits or expenses --but then the log-approximation becomes rather visibly inaccurate.