Logarithms and Exponential Functions
Properties of Exponents and Logarithms
It's time for our master-class before facing off with Expo and his minions. Log sends us off to an old, musty library with stack after stack of books. They've got titles like Ye Olde Mathematical Beasts and Logarithmica Adeptus. Whenever you open up one of the ancient books, dust puffs out all over your face. Don't sneeze, though, that'll just stir it up even more.
Looking through them we find some properties of logarithms and exponentials that will help you out. Some we've already met, some are fresh and new.
Sum of Logs: logbxy = logbx + logby
For all of these properties, looking at them in their exponential form is a big help. Let's do that:
e = logbxy is the same as be = xy
f + g = logb x + logb y is the same as bf × bg = xy
Remember that multiplying two numbers with the same base together will cause them to merge together and add the exponents together. For example 3 × 3 = 31 + 31 = 32, so bf + bg = be or bf + g.
Difference of Logs:
This one is similar to the previous property, but just in reverse gear. If two numbers with the same base are divided, their exponents are subtracted from each other.
Exponent in Log: logb xn = n logb x
Whoa, how'd that exponent just decide to go outside the log? Expo just doesn't care 'bout anyone now, it seems. Zip, zoom, fly—it's time for exponent mode.
be = xn
Imagine the following situation: 102 = 100, (n = 1).
If we cube each side, this is what would go down:
(102)3 = 1003
The power of 3 on the right side is n in both the logarithmic and exponential form. All you have to remember here is that when you raise an exponent to another exponent, you can multiply them:
102 × 3 = 1003
106 = 1003
Demonstrate the sum of logs by expanding log 100 and solving, then demonstrate the log in exponent property by expanding.
There are two other ways to represent log 100: log 10 × 10 or log 102.
If we expand the first way, we get log 10 + log 10 = 1 + 1 = 2. log 100 is also 2.
If we expand the second way, we get 2 log 10 = 2 × 1 = 2. No sweat.
Sample ProblemSimplify the following equation using the properties you've learned so far:
3 log4 16x5
First let's pull that exponent down from its pedestal:
3(5) log4 16x = 15 log4 16x
Then pull the logs apart, sure to hold onto the coefficient:
15 log4 16 + 15 log4 x
Simplify the first log, because
42 = 16: 15(2) + 15 log4 x = 30 + 15 log4 x
There's your answer:
30 + 15 log4 x.
Log of 1: logb 1 = 0
Essentially all this says is, "what exponent do we raise a base to so we get an answer of 1?" Because the base cannot be 1, that only leaves one answer: 0. Absolutely any base raised to the power of 0 is 1. The base could be 1,000,000, but raising it to the 0 power would still give us 1. How mystical.
Equal Base: logb b = 1
Again, let's put on our exponential-form goggles. (Now you look like you're on a secret mission. So classy.)
be = b
The logarithm answers the question, "What's the exponent?" If we need the two sides to equal, the only exponent that would give a correct answer would be one. It'd work out just to be b = b.
Rewrite the following logarithmic functions into a single log, simplifying where possible, using the properties you've learned.
y = log5 20x2 + 1/2 log5 25x – log5 5
Whoa boy. That's a Moby Dick-sized whale of a problem.
Let's go from left to right. First, pull out the exponent in the left log:
2 log5 x
Split the middle log into a sum:
Now that you've got all the skills to beat Expo at his own game, taking him down should be a cinch.