# 4.3 - 1 4.3 Logarithmic Functions Logarithms Logarithmic Equations Logarithmic Functions Properties...

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Slide 14.3 - *

4.3 - *

Logarithms

The previous section dealt with exponential functions of the form y = ax where a ≠ 1 and a>0. The horizontal line test shows that exponential functions are one-to-one, and thus have inverse functions.

Starting with y = ax and interchanging x and y yields x = ay .Here we need to solve for y

4.3 - *

4.3 - *

Logarithms

Here y is the exponent. We call this exponent a logarithm, symbolized by “log.” We wrte y=loga x . The number a is called the base of the logarithm, and x is called the argument of the expression. It is read “logarithm with base a of x,” or “logarithm of x with base a.”

4.3 - *

4.3 - *

For all real numbers y and all positive numbers a and x, where a ≠ 1,

if and only if

A logarithm is an exponent. The expression

loga x represents the exponent to which the base “a” must be raised in order to obtain x.

Logarithm

4.3 - *

4.3 - *

Logarithms

Exponent

Base

4.3 - *

4.3 - *

Power rule for exponents.

defines the logarithmic function with base a.

Logarithmic Function

Logarithmic Function

Exponential and logarithmic functions are inverses of each other. The graph of

y = 2x is shown in red. The graph of its inverse is found by reflecting the graph across the line y = x.

4.3 - *

4.3 - *

Logarithmic Function

The graph of the inverse function, defined by y = log2 x, shown in blue, has the y-axis as a vertical asymptote.

4.3 - *

4.3 - *

LOGARITHMIC FUNCTION

(x) = loga x, a > 1, is increasing and continuous on its entire domain, (0, ) .

For (x) = log2 x:

The y-axis is a vertical asymptote as x 0 from the right.

x

(x)

¼

– 2

½

– 1

1

0

2

1

4

2

8

3

4.3 - *

4.3 - *

LOGARITHMIC FUNCTION

(x) = loga x, 0 < a < 1, is decreasing and continuous on its entire domain, (0, ) .

For (x) = log1/2 x:

The y-axis is a vertical asymptote as x 0 from the right.

x

(x)

¼

2

½

1

1

0

2

– 1

4

– 2

8

– 3

4.3 - *

4.3 - *

Solution

a.

First graph y = (½)x . The graph of (x) = log1/2x is the reflection of the graph

y = (½)x across the line y = x.

4.3 - *

4.3 - *

Solution

a.

The graph of (x) = log2 (x – 1) is the graph of

(x) = log2 x translated 1 unit to the right. The vertical asymptote is x = 1. The domain of this function is (1, ) since logarithms can be found only for positive numbers. To find some ordered pairs to plot, use the equivalent exponential form of the equation

y = log2 (x – 1).

Solution

(– , ).

Solution

b.

The function defined by (x) = (log3 x) – 1 has the same graph as g(x) = log3 x translated 1 unit down. We find ordered pairs to plot by writing y = (log3 x) – 1

in exponential form.

Solution

b.

Again, choose y-values and calculate the corresponding x-values. The domain is (0, ) and the range is (– , ).

4.3 - *

4.3 - *

For x > 0, y > 0, a > 0, a ≠ 1, and any real number r:

Description

The logarithm of the product of two numbers is equal to the sum of the

logarithms of the numbers.

4.3 - *

4.3 - *

For x > 0, y > 0, a > 0, a ≠ 1, and any real number r:

Property

Description

The logarithm of the quotient of two numbers is equal to the difference between the logarithms of the numbers.

Properties of Logarithms

4.3 - *

4.3 - *

For x > 0, y > 0, a > 0, a ≠ 1, and any real number r:

Property

Description

The logarithm of a number raised to a power is equal to the exponent multiplied by the logarithm of the number.

Properties of Logarithms

Properties of Logarithms

Two additional properties of logarithms follow directly from the definition of loga x since a0 = 1 and a1 = a.

4.3 - *

4.3 - *

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

a.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

b.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

c.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

d.

Be careful with signs.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

e.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

f.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

f.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

a.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

b.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

c.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

c.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

c.

4.3 - *

4.3 - *

Caution There is no property of logarithms to rewrite a logarithm of a sum or difference. That is why, in Example 5(a),

log3(x + 2) was not written as log3 x + log3 2. Remember, log3 x + log3 2 = log3(x • 2).

The distributive property does not apply in a situation like this because log3 (x + y) is one term; “log” is a function name, not a factor.

4.3 - *

4.3 - *

Solution

a.

b.

4.3 - *

4.3 - *

Theorem on Inverses

By the results of this theorem,

.

a

xx

x

ya

4.3 - *

Logarithms

The previous section dealt with exponential functions of the form y = ax where a ≠ 1 and a>0. The horizontal line test shows that exponential functions are one-to-one, and thus have inverse functions.

Starting with y = ax and interchanging x and y yields x = ay .Here we need to solve for y

4.3 - *

4.3 - *

Logarithms

Here y is the exponent. We call this exponent a logarithm, symbolized by “log.” We wrte y=loga x . The number a is called the base of the logarithm, and x is called the argument of the expression. It is read “logarithm with base a of x,” or “logarithm of x with base a.”

4.3 - *

4.3 - *

For all real numbers y and all positive numbers a and x, where a ≠ 1,

if and only if

A logarithm is an exponent. The expression

loga x represents the exponent to which the base “a” must be raised in order to obtain x.

Logarithm

4.3 - *

4.3 - *

Logarithms

Exponent

Base

4.3 - *

4.3 - *

Power rule for exponents.

defines the logarithmic function with base a.

Logarithmic Function

Logarithmic Function

Exponential and logarithmic functions are inverses of each other. The graph of

y = 2x is shown in red. The graph of its inverse is found by reflecting the graph across the line y = x.

4.3 - *

4.3 - *

Logarithmic Function

The graph of the inverse function, defined by y = log2 x, shown in blue, has the y-axis as a vertical asymptote.

4.3 - *

4.3 - *

LOGARITHMIC FUNCTION

(x) = loga x, a > 1, is increasing and continuous on its entire domain, (0, ) .

For (x) = log2 x:

The y-axis is a vertical asymptote as x 0 from the right.

x

(x)

¼

– 2

½

– 1

1

0

2

1

4

2

8

3

4.3 - *

4.3 - *

LOGARITHMIC FUNCTION

(x) = loga x, 0 < a < 1, is decreasing and continuous on its entire domain, (0, ) .

For (x) = log1/2 x:

The y-axis is a vertical asymptote as x 0 from the right.

x

(x)

¼

2

½

1

1

0

2

– 1

4

– 2

8

– 3

4.3 - *

4.3 - *

Solution

a.

First graph y = (½)x . The graph of (x) = log1/2x is the reflection of the graph

y = (½)x across the line y = x.

4.3 - *

4.3 - *

Solution

a.

The graph of (x) = log2 (x – 1) is the graph of

(x) = log2 x translated 1 unit to the right. The vertical asymptote is x = 1. The domain of this function is (1, ) since logarithms can be found only for positive numbers. To find some ordered pairs to plot, use the equivalent exponential form of the equation

y = log2 (x – 1).

Solution

(– , ).

Solution

b.

The function defined by (x) = (log3 x) – 1 has the same graph as g(x) = log3 x translated 1 unit down. We find ordered pairs to plot by writing y = (log3 x) – 1

in exponential form.

Solution

b.

Again, choose y-values and calculate the corresponding x-values. The domain is (0, ) and the range is (– , ).

4.3 - *

4.3 - *

For x > 0, y > 0, a > 0, a ≠ 1, and any real number r:

Description

The logarithm of the product of two numbers is equal to the sum of the

logarithms of the numbers.

4.3 - *

4.3 - *

For x > 0, y > 0, a > 0, a ≠ 1, and any real number r:

Property

Description

The logarithm of the quotient of two numbers is equal to the difference between the logarithms of the numbers.

Properties of Logarithms

4.3 - *

4.3 - *

For x > 0, y > 0, a > 0, a ≠ 1, and any real number r:

Property

Description

The logarithm of a number raised to a power is equal to the exponent multiplied by the logarithm of the number.

Properties of Logarithms

Properties of Logarithms

Two additional properties of logarithms follow directly from the definition of loga x since a0 = 1 and a1 = a.

4.3 - *

4.3 - *

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

a.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

b.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

c.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

d.

Be careful with signs.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

e.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

f.

Rewrite each expression. Assume all variables represent positive real numbers, with a ≠ 1 and b ≠ 1.

Solution

f.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

a.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

b.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

c.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

c.

USING THE PROPERTIES OF LOGARITHMS

Write the expression as a single logarithm with coefficient 1. Assume all variables represent positive real numbers, with a ≠ 1and b ≠ 1.

Solution

c.

4.3 - *

4.3 - *

Caution There is no property of logarithms to rewrite a logarithm of a sum or difference. That is why, in Example 5(a),

log3(x + 2) was not written as log3 x + log3 2. Remember, log3 x + log3 2 = log3(x • 2).

The distributive property does not apply in a situation like this because log3 (x + y) is one term; “log” is a function name, not a factor.

4.3 - *

4.3 - *

Solution

a.

b.

4.3 - *

4.3 - *

Theorem on Inverses

By the results of this theorem,

.

a

xx

x

ya