# 6.3 Logarithmic functions  (Page 5/9)

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## Evaluating a natural logarithm using a calculator

Evaluate $\text{\hspace{0.17em}}y=\mathrm{ln}\left(500\right)\text{\hspace{0.17em}}$ to four decimal places using a calculator.

• Press [LN] .
• Enter $\text{\hspace{0.17em}}500,$ followed by [ ) ] .
• Press [ENTER] .

Rounding to four decimal places, $\text{\hspace{0.17em}}\mathrm{ln}\left(500\right)\approx 6.2146$

Evaluate $\text{\hspace{0.17em}}\mathrm{ln}\left(-500\right).$

It is not possible to take the logarithm of a negative number in the set of real numbers.

Access this online resource for additional instruction and practice with logarithms.

## Key equations

 Definition of the logarithmic function For if and only if Definition of the common logarithm For if and only if Definition of the natural logarithm For if and only if

## Key concepts

• The inverse of an exponential function is a logarithmic function, and the inverse of a logarithmic function is an exponential function.
• Logarithmic equations can be written in an equivalent exponential form, using the definition of a logarithm. See [link] .
• Exponential equations can be written in their equivalent logarithmic form using the definition of a logarithm See [link] .
• Logarithmic functions with base $\text{\hspace{0.17em}}b\text{\hspace{0.17em}}$ can be evaluated mentally using previous knowledge of powers of $\text{\hspace{0.17em}}b.\text{\hspace{0.17em}}$ See [link] and [link] .
• Common logarithms can be evaluated mentally using previous knowledge of powers of $\text{\hspace{0.17em}}10.\text{\hspace{0.17em}}$ See [link] .
• When common logarithms cannot be evaluated mentally, a calculator can be used. See [link] .
• Real-world exponential problems with base $\text{\hspace{0.17em}}10\text{\hspace{0.17em}}$ can be rewritten as a common logarithm and then evaluated using a calculator. See [link] .
• Natural logarithms can be evaluated using a calculator [link] .

## Verbal

What is a base $\text{\hspace{0.17em}}b\text{\hspace{0.17em}}$ logarithm? Discuss the meaning by interpreting each part of the equivalent equations $\text{\hspace{0.17em}}{b}^{y}=x\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}{\mathrm{log}}_{b}x=y\text{\hspace{0.17em}}$ for $\text{\hspace{0.17em}}b>0,b\ne 1.$

A logarithm is an exponent. Specifically, it is the exponent to which a base $\text{\hspace{0.17em}}b\text{\hspace{0.17em}}$ is raised to produce a given value. In the expressions given, the base $\text{\hspace{0.17em}}b\text{\hspace{0.17em}}$ has the same value. The exponent, $\text{\hspace{0.17em}}y,$ in the expression $\text{\hspace{0.17em}}{b}^{y}\text{\hspace{0.17em}}$ can also be written as the logarithm, $\text{\hspace{0.17em}}{\mathrm{log}}_{b}x,$ and the value of $\text{\hspace{0.17em}}x\text{\hspace{0.17em}}$ is the result of raising $\text{\hspace{0.17em}}b\text{\hspace{0.17em}}$ to the power of $\text{\hspace{0.17em}}y.$

How is the logarithmic function $\text{\hspace{0.17em}}f\left(x\right)={\mathrm{log}}_{b}x\text{\hspace{0.17em}}$ related to the exponential function $\text{\hspace{0.17em}}g\left(x\right)={b}^{x}?\text{\hspace{0.17em}}$ What is the result of composing these two functions?

How can the logarithmic equation $\text{\hspace{0.17em}}{\mathrm{log}}_{b}x=y\text{\hspace{0.17em}}$ be solved for $\text{\hspace{0.17em}}x\text{\hspace{0.17em}}$ using the properties of exponents?

Since the equation of a logarithm is equivalent to an exponential equation, the logarithm can be converted to the exponential equation $\text{\hspace{0.17em}}{b}^{y}=x,$ and then properties of exponents can be applied to solve for $\text{\hspace{0.17em}}x.$

Discuss the meaning of the common logarithm. What is its relationship to a logarithm with base $\text{\hspace{0.17em}}b,$ and how does the notation differ?

Discuss the meaning of the natural logarithm. What is its relationship to a logarithm with base $\text{\hspace{0.17em}}b,$ and how does the notation differ?

The natural logarithm is a special case of the logarithm with base $\text{\hspace{0.17em}}b\text{\hspace{0.17em}}$ in that the natural log always has base $\text{\hspace{0.17em}}e.\text{\hspace{0.17em}}$ Rather than notating the natural logarithm as $\text{\hspace{0.17em}}{\mathrm{log}}_{e}\left(x\right),$ the notation used is $\text{\hspace{0.17em}}\mathrm{ln}\left(x\right).$

## Algebraic

For the following exercises, rewrite each equation in exponential form.

${\text{log}}_{4}\left(q\right)=m$

${\text{log}}_{a}\left(b\right)=c$

${a}^{c}=b$

${\mathrm{log}}_{16}\left(y\right)=x$

${\mathrm{log}}_{x}\left(64\right)=y$

${x}^{y}=64$

${\mathrm{log}}_{y}\left(x\right)=-11$

f(x)=x/x+2 given g(x)=1+2x/1-x show that gf(x)=1+2x/3
proof
AUSTINE
sebd me some questions about anything ill solve for yall
how to solve x²=2x+8 factorization?
x=2x+8 x-2x=2x+8-2x x-2x=8 -x=8 -x/-1=8/-1 x=-8 prove: if x=-8 -8=2(-8)+8 -8=-16+8 -8=-8 (PROVEN)
Manifoldee
x=2x+8
Manifoldee
×=2x-8 minus both sides by 2x
Manifoldee
so, x-2x=2x+8-2x
Manifoldee
then cancel out 2x and -2x, cuz 2x-2x is obviously zero
Manifoldee
so it would be like this: x-2x=8
Manifoldee
then we all know that beside the variable is a number (1): (1)x-2x=8
Manifoldee
so we will going to minus that 1-2=-1
Manifoldee
so it would be -x=8
Manifoldee
so next step is to cancel out negative number beside x so we get positive x
Manifoldee
so by doing it you need to divide both side by -1 so it would be like this: (-1x/-1)=(8/-1)
Manifoldee
so -1/-1=1
Manifoldee
so x=-8
Manifoldee
Manifoldee
so we should prove it
Manifoldee
x=2x+8 x-2x=8 -x=8 x=-8 by mantu from India
mantu
lol i just saw its x²
Manifoldee
x²=2x-8 x²-2x=8 -x²=8 x²=-8 square root(x²)=square root(-8) x=sq. root(-8)
Manifoldee
I mean x²=2x+8 by factorization method
Kristof
I think x=-2 or x=4
Kristof
x= 2x+8 ×=8-2x - 2x + x = 8 - x = 8 both sides divided - 1 -×/-1 = 8/-1 × = - 8 //// from somalia
Mohamed
hii
Amit
how are you
Dorbor
well
Biswajit
can u tell me concepts
Gaurav
Find the possible value of 8.5 using moivre's theorem
which of these functions is not uniformly cintinuous on (0, 1)? sinx
which of these functions is not uniformly continuous on 0,1
solve this equation by completing the square 3x-4x-7=0
X=7
Muustapha
=7
mantu
x=7
mantu
3x-4x-7=0 -x=7 x=-7
Kr
x=-7
mantu
9x-16x-49=0 -7x=49 -x=7 x=7
mantu
what's the formula
Modress
-x=7
Modress
new member
siame
what is trigonometry
deals with circles, angles, and triangles. Usually in the form of Soh cah toa or sine, cosine, and tangent
Thomas
solve for me this equational y=2-x
what are you solving for
Alex
solve x
Rubben
you would move everything to the other side leaving x by itself. subtract 2 and divide -1.
Nikki
then I got x=-2
Rubben
it will b -y+2=x
Alex
goodness. I'm sorry. I will let Alex take the wheel.
Nikki
ouky thanks braa
Rubben
I think he drive me safe
Rubben
how to get 8 trigonometric function of tanA=0.5, given SinA=5/13? Can you help me?m
More example of algebra and trigo
What is Indices
If one side only of a triangle is given is it possible to solve for the unkown two sides?
cool
Rubben
kya
Khushnama
please I need help in maths
Okey tell me, what's your problem is?
Navin