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Common and Natural Logarithms

In this lesson, we will learn common logarithms and natural logarithms and how to solve problems using common log and natural log.

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Common Logarithms

Logarithms to base 10 are called common logarithms. We often write “log10” as “log” or “lg”. Common logarithms can be evaluated using a scientific calculator.

Recall that by the definition of logarithm.

log Y = X Y = 10X

Natural Logarithms

Besides base 10, another important base is e. Log to base e are called natural logarithms. “loge” are often abbreviated as “ln”. Natural logarithms can also be evaluated using a scientific calculator.

By definition

ln Y = XY = eX

Using a calculator, we can use common and natural logarithms to solve equations of the form ax = b, especially when b cannot be expressed as an.


Solve the equations
a) 6x + 2 = 21
b) e2x = 9


a) 6x + 2 = 21
log 6x + 2 = log 21
(x + 2) log 6 = log 21

b) e3x = 9
ln e3x = ln 9
3x ln e = ln 9
3x = ln 9


Express 3x(22x) = 7(5x) in the form ax = b. Hence, find x.


Since 3x(22x) = 3x(22)x = (3 × 4)x = 12x

the equation becomes

12x = 7(5x)


Common and Natural Logarithms
We can use many bases for a logarithm, but the bases most typically used are the bases of the common logarithm and the natural logarithm. The common logarithm has base 10, and is represented on the calculator as log(x). The natural logarithm has base e, a famous irrational number, and is represented on the calculator by ln(x). The natural and common logarithm can be found throughout Algebra and Calculus.

Defines common log, log x, and natural log, ln x, and works through examples and problems using a calculator.

Common and Natural Logarithms.
Properties of Logarithms

Common and Natural Logs
This video models how to solve 2 equations. The first example is with common logs and the second example is natural logs. It is good to remember the properties of logarithms also can be applied to natural logs.

Rotate to landscape screen format on a mobile phone or small tablet to use the Mathway widget, a free math problem solver that answers your questions with step-by-step explanations.

You can use the free Mathway calculator and problem solver below to practice Algebra or other math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.

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