The reverse process of expanding logarithms is called combining or condensing logarithmic expressions into a single quantity. Other textbooks refer to this as simplifying logarithms. But, they all mean the same.
The idea is that you are given a bunch of log expressions as sums and/or differences, and your task is to put them back or compress them into a “nice” one log expression.
I highly recommend that you review the rules of logarithms first before looking at the worked examples belowbecause you’ll use them in reverse.
For instance, if you go from left to right of the equation then youmust be expanding, while going from right to left then you must be condensing.
Rules of Logarithms
Study the description of each ruleto get an intuitive understanding of it.
Description of Each Logarithm Rule
Rule1: Product Rule
The logarithm of the product of numbers is the sum of the logarithms of individual numbers.
Rule 2: Quotient Rule
The logarithm of the quotient of numbers is the difference of the logarithm of individual numbers.
Rule 3: Power Rule
The logarithm of an exponential number is the exponent times the logarithm of the base.
Rule 4: Zero Rule
The logarithm of [latex]1[/latex] with [latex]b > 0[/latex] but [latex]b \ne 1[/latex] equals zero.
Rule 5: Identity Rule
The logarithm of a number that is equal to its base is just [latex]1[/latex].
Rule 6: Log of Exponent Rule
The logarithm of an exponential number where its base is the same as the base of the log equals the exponent.
Rule 7: Exponent of Log Rule
Raising the logarithm of a number to its base equals the number.
Examples of How to Combine or Condense Logarithms
Example 1: Combine or condense the following log expressions into a single logarithm:
This is theProduct Rule in reversebecause they are the sum of log expressions. That means we canconvert those addition operations (plus symbols) outside into multiplication inside.
Since we have “condensed” or “compressed” three logarithmic expressions into one log expression, then that should be our final answer.
Example 2: Combine or condense the following log expressions into a single logarithm:
The difference between logarithmic expressions implies the Quotient Rule. I can put together that variable [latex]x[/latex] and constant [latex]2[/latex] inside a single parenthesis using division operation.
Example 3: Combine or condense the following log expressions into a single logarithm:
Start by applying Rule 2 (Power Rule) in reverse to take care of the constants or numbers on the left of the logs. Remember that the Power Rule brings down the exponent, so the opposite direction is to put it up.
The next step is to use the Productand Quotient rules from left to right. This is how it looks when you solve it.
Example 4: Combine or condense the following log expressions into a single logarithm:
I can apply the reverse of the Power rule to place the exponents on variable [latex]x[/latex] for the two expressions and leave the third one for now because it is already fine. Next, utilize the Product Rule to deal with the plus symbol followed by the Quotient Rule to address the subtraction part.
In this problem, watch out for the opportunity where you will multiply and divide exponential expressions. Just a reminder, you add the exponents during multiplication and subtract during division.
Example 5: Combine or condense the following log expressions into a single logarithm:
I suggest that you don’t skip any steps. Unnecessary errors can be prevented by being careful and methodical in every step. Check and recheck your work to ensure that you don’t miss any important opportunity to simplify the expressions further such as combining exponential expressions with the same base.
So for this one, start with the first log expression by applying the Power Rule to address that coefficient of [latex]\large{1 \over 2}[/latex]. Next, think of the power [latex]\large{1 \over 2}[/latex] as a square root operation. The square root can definitely simplify the perfect square [latex]81[/latex] and the [latex]{y^{12}}[/latex] because it has an even power.
Example 6: Combine or condense the following log expressions into a single logarithm:
The steps involved are very similar to previous problems but there’s a “trick” that you need to pay attention to. This is an interesting problem because of the constant [latex]3[/latex]. We have to rewrite [latex]3[/latex] in the logarithmic form such that it has a base of [latex]4[/latex]. To construct it, use Rule 5 (Identity Rule) in reverse because it makes sense that [latex]3 = {\log _4}\left( {{4^3}} \right)[/latex].
You may also be interested in these related math lessons or tutorials:
Expanding Logarithms
Logarithm Explained
Logarithm Rules
Solving Logarithmic Equations
Proofs of Logarithm Properties or Rules
