3.8: Common and Natural Logarithms
By now, you know that can be solved if you recognize that . What about numbers that aren't so 'clean'? There aren't many people who could calculate the answer to in their head! It would be great to use a calculator, but most only have two log functions: base 10 and base e .
Is there a way to convert from one base to another, so we can use a calculator?
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 Brightstorm: Algebra 2  Common and Natural Logarithms
Guidance
Although a log function can have any positive number as a base, there are really only two bases that are commonly used in the real world. Both may be written without a base noted, like: , so you may need to use the context to decide which is appropriate.
The common log is a log with base 10. It is used to define pH, earthquake magnitude, and sound decibel levels, among many many other common realworld values.
The natural log , sometimes written , is a log with base e . The transcendental number e is approximately 2.71828 and is used in any number of calculations involving constant growth in chemistry, physics, biology, finance, etc..
Using a calculator for logs
You may have noticed that the common log and the natural log are the only log buttons on your calculator. We can use either the common log or the natural log to find the values of logs with other bases.
The equation is called the change of base formula, and may be used to convert to common log or natural log .
You may also see the change of base formula as , which is the same formula specifying a conversion to the natural log .
Using the change of base formula, we can find the common log (or the natural log) equivalent of any other base so that we can use a calculator to find the value of an expression.
Consider log _{ 3 } 35. If we use the change of base formula to convert to base 10, and then the button on a calculator, we find that

 .
Example A
Evaluate each log
a.  b.  c. 

Solution:
Remember that (with no base specified) commonly refers to :
 a. because .
 b. because
 c. because
Example B
For each log value, determine two integers between which the log value should lie. Then use a calculator to find the value of the log.
a. log 50  b. log 818 

Solution:
 a. log 50

 The value of this log should be between 1 and 2, as 10 ^{ 1 } = 10, and 10 ^{ 2 } = 100.
 Using a calculator, you should find that log 50 ≈ 1.698970004.
 b. log 818

 The value of this log should be between 2 and 3, as 10 ^{ 2 } = 100, and 10 ^{ 3 } = 1000.

 Using a calculator, you should find that log 818 ≈ 2.912753304.
Example C
Estimate the value, and then use the change of base formula to find the value of .
Solution:
is close to 4 because and . Using the change of base formula, we have . Using a calculator, you should find that the approximate value of this expression is 4.087462841.
Concept question wrapup You should have no problem with the question from the beginning of the lesson now: . Using the change of base formula: Using a calculator to find the common logs of 247 and 7, we get (approximately): . We can verify with:


Vocabulary
Common Logarithm: A common logarithm is a log with base 10 k . The log is usually written without the base.
Natural Logarithm: A natural log is a log with base e . The natural log is written as ln.
Trancendental Number: A number that is not the root of any rational polynomial function. Examples include and .
(constant): A constant used as the base of natural logarithms , equal to approximately 2.71828.
Guided Practice
Questions
1) Find the value of each natural log.
a.  b. 

2) Solve the equation:
3) Solve the equation:
4) Find the value:
5) Find the value:
Answers
1) a. is between 4 and 5. You can estimate this by rounding up to 3, and considering powers of 3:

 and

 Using a calculator, you should find that ≈ 4.605171086.
 b. Recall that a square root is the same as an exponent of 1/2. Therefore
2) To solve: :
 : Take the log of both sides.
 : Using
 : Divide both sides by and add .
 : With a calculator
 : With a calculator
3) To solve: :
 : Rule of exponents
 : By multiplication
 : Divide both sides by
 : Take the log of both sides
 : Using
 : Divide both sides by
 : With a calculator
4) Use a calculator to find the values:
 and
5) Recall that a without a specified base is commonly :
 and : With a calculator
Practice
 What is a common logarithm? Where are common logs most commonly used?
 What is a natural logarithm? Where are natural logs commonly used?
Evaluate each expression:
Convert to a common logarithm and evaluate:
Convert to a natural logarithm and evaluate:
Find the values of the natural logarithms:
Convert the Natural Logs to exponential form, and solve.
 If and then
 If then
 If then
 If then
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Description
Learning Objectives
Here you will explore the common logarithm, which is a log base 10, and the natural logarithm, which is a log base e . You will also learn how to convert from one base to another when working with logs.