# 4.3 Logarithmic functions  (Page 4/9)

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## Finding the value of a common logarithm mentally

Evaluate $\text{\hspace{0.17em}}y=\mathrm{log}\left(1000\right)\text{\hspace{0.17em}}$ without using a calculator.

First we rewrite the logarithm in exponential form: $\text{\hspace{0.17em}}{10}^{y}=1000.\text{\hspace{0.17em}}$ Next, we ask, “To what exponent must $\text{\hspace{0.17em}}10\text{\hspace{0.17em}}$ be raised in order to get 1000?” We know

${10}^{3}=1000$

Therefore, $\text{\hspace{0.17em}}\mathrm{log}\left(1000\right)=3.$

Evaluate $\text{\hspace{0.17em}}y=\mathrm{log}\left(1,000,000\right).$

$\mathrm{log}\left(1,000,000\right)=6$

Given a common logarithm with the form $\text{\hspace{0.17em}}y=\mathrm{log}\left(x\right),$ evaluate it using a calculator.

1. Press [LOG] .
2. Enter the value given for $\text{\hspace{0.17em}}x,$ followed by [ ) ] .
3. Press [ENTER] .

## Finding the value of a common logarithm using a calculator

Evaluate $\text{\hspace{0.17em}}y=\mathrm{log}\left(321\right)\text{\hspace{0.17em}}$ to four decimal places using a calculator.

• Press [LOG] .
• Enter 321 , followed by [ ) ] .
• Press [ENTER] .

Rounding to four decimal places, $\text{\hspace{0.17em}}\mathrm{log}\left(321\right)\approx 2.5065.$

Evaluate $\text{\hspace{0.17em}}y=\mathrm{log}\left(123\right)\text{\hspace{0.17em}}$ to four decimal places using a calculator.

$\mathrm{log}\left(123\right)\approx 2.0899$

## Rewriting and solving a real-world exponential model

The amount of energy released from one earthquake was 500 times greater than the amount of energy released from another. The equation $\text{\hspace{0.17em}}{10}^{x}=500\text{\hspace{0.17em}}$ represents this situation, where $\text{\hspace{0.17em}}x\text{\hspace{0.17em}}$ is the difference in magnitudes on the Richter Scale. To the nearest thousandth, what was the difference in magnitudes?

We begin by rewriting the exponential equation in logarithmic form.

Next we evaluate the logarithm using a calculator:

• Press [LOG] .
• Enter $\text{\hspace{0.17em}}500,$ followed by [ ) ] .
• Press [ENTER] .
• To the nearest thousandth, $\text{\hspace{0.17em}}\mathrm{log}\left(500\right)\approx 2.699.$

The difference in magnitudes was about $\text{\hspace{0.17em}}2.699.$

The amount of energy released from one earthquake was $\text{\hspace{0.17em}}\text{8,500}\text{\hspace{0.17em}}$ times greater than the amount of energy released from another. The equation $\text{\hspace{0.17em}}{10}^{x}=8500\text{\hspace{0.17em}}$ represents this situation, where $\text{\hspace{0.17em}}x\text{\hspace{0.17em}}$ is the difference in magnitudes on the Richter Scale. To the nearest thousandth, what was the difference in magnitudes?

The difference in magnitudes was about $\text{\hspace{0.17em}}3.929.$

## Using natural logarithms

The most frequently used base for logarithms is $\text{\hspace{0.17em}}e.\text{\hspace{0.17em}}$ Base $\text{\hspace{0.17em}}e\text{\hspace{0.17em}}$ logarithms are important in calculus and some scientific applications; they are called natural logarithms . The base $\text{\hspace{0.17em}}e\text{\hspace{0.17em}}$ logarithm, $\text{\hspace{0.17em}}{\mathrm{log}}_{e}\left(x\right),$ has its own notation, $\text{\hspace{0.17em}}\mathrm{ln}\left(x\right).$

Most values of $\text{\hspace{0.17em}}\mathrm{ln}\left(x\right)\text{\hspace{0.17em}}$ can be found only using a calculator. The major exception is that, because the logarithm of 1 is always 0 in any base, $\text{\hspace{0.17em}}\mathrm{ln}1=0.\text{\hspace{0.17em}}$ For other natural logarithms, we can use the $\text{\hspace{0.17em}}\mathrm{ln}\text{\hspace{0.17em}}$ key that can be found on most scientific calculators. We can also find the natural logarithm of any power of $\text{\hspace{0.17em}}e\text{\hspace{0.17em}}$ using the inverse property of logarithms.

## Definition of the natural logarithm

A natural logarithm    is a logarithm with base $\text{\hspace{0.17em}}e.$ We write ${\mathrm{log}}_{e}\left(x\right)$ simply as $\mathrm{ln}\left(x\right).$ The natural logarithm of a positive number $x$ satisfies the following definition.

For $\text{\hspace{0.17em}}x>0,$

We read $\text{\hspace{0.17em}}\mathrm{ln}\left(x\right)\text{\hspace{0.17em}}$ as, “the logarithm with base $\text{\hspace{0.17em}}e\text{\hspace{0.17em}}$ of $\text{\hspace{0.17em}}x$ ” or “the natural logarithm of $\text{\hspace{0.17em}}x.$

The logarithm $\text{\hspace{0.17em}}y\text{\hspace{0.17em}}$ is the exponent to which $\text{\hspace{0.17em}}e\text{\hspace{0.17em}}$ must be raised to get $\text{\hspace{0.17em}}x.$

Since the functions $\text{\hspace{0.17em}}y=e{}^{x}\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}y=\mathrm{ln}\left(x\right)\text{\hspace{0.17em}}$ are inverse functions, $\text{\hspace{0.17em}}\mathrm{ln}\left({e}^{x}\right)=x\text{\hspace{0.17em}}$ for all $\text{\hspace{0.17em}}x\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}e{}^{\mathrm{ln}\left(x\right)}=x\text{\hspace{0.17em}}$ for $\text{\hspace{0.17em}}x>0.$

Given a natural logarithm with the form $\text{\hspace{0.17em}}y=\mathrm{ln}\left(x\right),$ evaluate it using a calculator.

1. Press [LN] .
2. Enter the value given for $\text{\hspace{0.17em}}x,$ followed by [ ) ] .
3. Press [ENTER] .

The average annual population increase of a pack of wolves is 25.
how do you find the period of a sine graph
Period =2π if there is a coefficient (b), just divide the coefficient by 2π to get the new period
Am
if not then how would I find it from a graph
Imani
by looking at the graph, find the distance between two consecutive maximum points (the highest points of the wave). so if the top of one wave is at point A (1,2) and the next top of the wave is at point B (6,2), then the period is 5, the difference of the x-coordinates.
Am
you could also do it with two consecutive minimum points or x-intercepts
Am
I will try that thank u
Imani
Case of Equilateral Hyperbola
ok
Zander
ok
Shella
f(x)=4x+2, find f(3)
Benetta
f(3)=4(3)+2 f(3)=14
lamoussa
14
Vedant
pre calc teacher: "Plug in Plug in...smell's good" f(x)=14
Devante
8x=40
Chris
Explain why log a x is not defined for a < 0
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Momo
how can are find the domain and range of a relations
the range is twice of the natural number which is the domain
Morolake
A cell phone company offers two plans for minutes. Plan A: $15 per month and$2 for every 300 texts. Plan B: $25 per month and$0.50 for every 100 texts. How many texts would you need to send per month for plan B to save you money?
6000
Robert
more than 6000
Robert
can I see the picture
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with doing calculus
SLIMANE
Thanks po.
Jenica
Hey I am new to precalculus, and wanted clarification please on what sine is as I am floored by the terms in this app? I don't mean to sound stupid but I have only completed up to college algebra.
I don't know if you are looking for a deeper answer or not, but the sine of an angle in a right triangle is the length of the opposite side to the angle in question divided by the length of the hypotenuse of said triangle.
Marco
can you give me sir tips to quickly understand precalculus. Im new too in that topic. Thanks
Jenica
if you remember sine, cosine, and tangent from geometry, all the relationships are the same but they use x y and r instead (x is adjacent, y is opposite, and r is hypotenuse).
Natalie
it is better to use unit circle than triangle .triangle is only used for acute angles but you can begin with. Download any application named"unit circle" you find in it all you need. unit circle is a circle centred at origine (0;0) with radius r= 1.
SLIMANE
What is domain
johnphilip
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what is foci?
This term is plural for a focus, it is used for conic sections. For more detail or other math questions. I recommend researching on "Khan academy" or watching "The Organic Chemistry Tutor" YouTube channel.
Chris
how to determine the vertex,focus,directrix and axis of symmetry of the parabola by equations