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Logarithm Rules

In my previous post, I introduced the concept of logarithms to you. I explained how logarithms and exponents are connected, and then showed you a quick trick to help you remember how to convert between the two. Now in this post, I’m going to go a little bit deeper and explain a few rules of logarithms to help you actually do math with them. Considering how you now already know that logs and exponents are related, it should come as no surprise that, just like there is a set of exponent rules, there is also a set of logarithmic rules.

As you look at these logarithm rules, keep in mind that by convention, if you write logs without the subscript number to indicate their base, it is assumed that you are dealing in base–10. For simplicity, this is the convention that I am going to use in this post, though these rules certainly apply when dealing with logs of other bases.

With that intro out of the way, let’s get to it.

The first law of logarithms is the product rule. If you are familiar with the product rule of exponents, then this logarithm law should be a piece of cake for you. Where the exponent rule says that when multiplying exponential expressions with the same base, you simply add the exponents, this same thing applies when multiplying logarithms of the same base. Therefore, the rule states that the logarithm of a product is equal to the sum of the logarithms.

This rule is very commonly used, and it is important to recognize that you can use it in either direction. That is, the logarithm of a product converts to a sum of logarithms, and vice versa.

The next logarithmic law is the quotient rule. Again, this law can easily be derived by applying your knowledge of the exponent quotient rule (though I will leave that for you as an exercise). However, it does appear to look different. This rule states that the logarithm of a quotient is equal to the logarithm of the numerator minus the logarithm of the denominator. Sounds like a mouthful, but the expression is probably much simpler to understand.

Again, watch for opportunities to use this relationship in either direction!

The third law of logs is the power rule. This one is surprisingly simple to remember, and again can be found by manipulating exponent and logarithm laws. Quite simply, this law says that when you have a logarithm of an exponential expression, the exponent can be “brought out” of the log and used as a coefficient for the log.

The last of the rules of logarithms that I’m going to discuss here today is called the base change rule. Recall that I stated above that all of my examples that I’ve used so far in this post use the convention of an assumed base–10. If I wanted to change my expression to utilize a different base, this rule helps us to do that. So then, if I have my log in some base of a number, and I want to express this in terms of a different base, I simply take the log in my new base of the original number and divide that by the log in my new base of the original base. Sounds wordy, but again, a picture is worth a thousand words:

Here, my original base is B, and my new base that I want to express things in is X.

That is all I have to say about the rules of logarithms in this short introduction to them. They are fairly straight forward themselves, though can be used in very complex equations. I will try to do a separate post soon outlining some examples of all of these rules, though I do think that the general forms that I’ve noted above are pretty self-explanatory.

If you are interested in learning more about logarithms, there is a much more thorough summary of logarithms at the Learning and Teaching Math blog, which I highly recommend (for this and other math topics!)

Logarithms Explained – What is a Logarithm?

Logarithms sound like very advanced math concepts, but they’re not really that much more difficult than many other mathematical concepts. The word “logarithm,” like the word “algorithm,” gives the impression of being a very complex topic. However, much like how an algorithm is nothing more complicated than a set of rules or processes to follow, a logarithm can also be very easy to work out. The first step to clearing up any confusion, though, is to explain what is a logarithm?

So then, just what is a logarithm?

A logarithm is related to exponents. Actually, it’s kind of like working backwards with exponents. A logarithm (or “log” for short) of a number is the exponent that another value (the base) must be raised to equal that number. Still sounds complicated, right? Looking at it another way, a logarithm tells us how many of one number we multiply together to get another number. That sounds a bit more manageable. Now let’s look at an example so that you can really see how easy this is.

Let’s consider the number 81. How many 3’s do we multiply together to equal 81?
3 x 3 = 9
3 x 3 x 3 = 27
3 x 3 x 3 x 3 = 81
As you can see, and easily work out yourself, you multiply 3 four times to produce 81. Another way to write this is with exponents, like this:

Now, what does any of this have to do with logarithms? In this example, we can say that the log with base 3 of 81 is 4. Or, as we found above, we multiply 3 (our base) 4 times to get 81. It is more common to say this as “the base 3 log of 81 is 4.”

What would a new math concept be without introducing a new expression? Of course there is a simple way of writing this down on paper, rather than longhand with words. Here is what our expression looks like:

Let’s compare our two expressions (the exponential notation and the log notation), and then I will show you a trick to help you work with them. This trick got me through years of logs in school! Notice first that all of the numbers are used in both expressions, though they’re in different places. In this case, the 3 is the base value, which is raised by the exponent 4 to produce the result 81. Similarly, with the logarithm notation, the base value is again 3, and we want to know what number this base is raised to give our results of 81. So, as you can see, the solution to the logarithm actually tells us what the exponent is!

Here is the simple arrangement of these components that help when working with logs. It can basically be boiled down to a simple rearrangement. The 3 moves over and bumps the 4 up to the exponent level, you remove the “log” word, and you’re left with your exponential notation of the expression.

Hopefully this post has helped you to understand what a logarithm is, and how logarithms are related to exponents. As I explained at the start, the concept is not that difficult. Now, much like the exponents they are related to, there are a bunch of rules that you need to follow when doing any kind of mathematical operations involving them. In my next post, I’ll begin to explain some of these logarithm rules.

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