4.5 Logarithmic properties (Page 2/10)

Precalculus

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\log_{b} (M N) = \log_{b} (M) + \log_{b} (N) .

Let $m = \log_{b} M$ and $n = \log_{b} N .$ In exponential form, these equations are $b^{m} = M$ and $b^{n} = N .$ It follows that

\begin{array}{l} \log_{b} (M N) & = \log_{b} (b^{m} b^{n}) & Substitute for M and N . \\ = \log_{b} (b^{m + n}) & Apply the product rule for exponents . \\ = m + n & Apply the inverse property of logs . \\ = \log_{b} (M) + \log_{b} (N) & Substitute for m and n . \end{array}

Note that repeated applications of the product rule for logarithms allow us to simplify the logarithm of the product of any number of factors. For example, consider $\log_{b} (w x y z) .$ Using the product rule for logarithms, we can rewrite this logarithm of a product as the sum of logarithms of its factors:

\log_{b} (w x y z) = \log_{b} w + \log_{b} x + \log_{b} y + \log_{b} z

A General Note

The product rule for logarithms

The product rule for logarithms can be used to simplify a logarithm of a product by rewriting it as a sum of individual logarithms.

\log_{b} (M N) = \log_{b} (M) + \log_{b} (N) for b > 0

How To

Given the logarithm of a product, use the product rule of logarithms to write an equivalent sum of logarithms.

Factor the argument completely, expressing each whole number factor as a product of primes.
Write the equivalent expression by summing the logarithms of each factor.

Using the product rule for logarithms

Expand $\log_{3} (30 x (3 x + 4)) .$

We begin by factoring the argument completely, expressing $30$ as a product of primes.

\log_{3} (30 x (3 x + 4)) = \log_{3} (2 \cdot 3 \cdot 5 \cdot x \cdot (3 x + 4))

Next we write the equivalent equation by summing the logarithms of each factor.

\log_{3} (30 x (3 x + 4)) = \log_{3} (2) + \log_{3} (3) + \log_{3} (5) + \log_{3} (x) + \log_{3} (3 x + 4)

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Expand $\log_{b} (8 k) .$

$\log_{b} 2 + \log_{b} 2 + \log_{b} 2 + \log_{b} k = 3 \log_{b} 2 + \log_{b} k$

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Using the quotient rule for logarithms

For quotients, we have a similar rule for logarithms. Recall that we use the quotient rule of exponents to combine the quotient of exponents by subtracting: $x^{\frac{a}{b}} = x^{a - b} .$ The quotient rule for logarithms says that the logarithm of a quotient is equal to a difference of logarithms. Just as with the product rule, we can use the inverse property to derive the quotient rule.

Given any real number $x$ and positive real numbers $M,$ $N,$ and $b,$ where $b \neq 1,$ we will show

\log_{b} (\frac{M}{N}) = \log_{b} (M) - \log_{b} (N) .

Let $m = \log_{b} M$ and $n = \log_{b} N .$ In exponential form, these equations are $b^{m} = M$ and $b^{n} = N .$ It follows that

\begin{array}{l} \log_{b} (\frac{M}{N}) & = \log_{b} (\frac{b^{m}}{b^{n}}) & Substitute for M and N . \\ = \log_{b} (b^{m - n}) & Apply the quotient rule for exponents . \\ = m - n & Apply the inverse property of logs . \\ = \log_{b} (M) - \log_{b} (N) & Substitute for m and n . \end{array}

For example, to expand $\log (\frac{2 x^{2} + 6 x}{3 x + 9}),$ we must first express the quotient in lowest terms. Factoring and canceling we get,

\begin{array}{l} \log (\frac{2 x^{2} + 6 x}{3 x + 9}) = \log (\frac{2 x (x + 3)}{3 (x + 3)}) & Factor the numerator and denominator . \\ = \log (\frac{2 x}{3}) & Cancel the common factors . \end{array}

Next we apply the quotient rule by subtracting the logarithm of the denominator from the logarithm of the numerator. Then we apply the product rule.

\begin{array}{l} \log (\frac{2 x}{3}) = \log (2 x) - \log (3) \\ = \log (2) + \log (x) - \log (3) \end{array}

A General Note

The quotient rule for logarithms

The quotient rule for logarithms can be used to simplify a logarithm or a quotient by rewriting it as the difference of individual logarithms.

\log_{b} (\frac{M}{N}) = \log_{b} M - \log_{b} N

How To

Given the logarithm of a quotient, use the quotient rule of logarithms to write an equivalent difference of logarithms.

Express the argument in lowest terms by factoring the numerator and denominator and canceling common terms.
Write the equivalent expression by subtracting the logarithm of the denominator from the logarithm of the numerator.
Check to see that each term is fully expanded. If not, apply the product rule for logarithms to expand completely.

Questions & Answers

Three charges q_{1}=+3\mu C, q_{2}=+6\mu C and q_{3}=+8\mu C are located at (2,0)m (0,0)m and (0,3) coordinates respectively. Find the magnitude and direction acted upon q_{2} by the two other charges.Draw the correct graphical illustration of the problem above showing the direction of all forces.

Kate Reply

To solve this problem, we need to first find the net force acting on charge q_{2}. The magnitude of the force exerted by q_{1} on q_{2} is given by F=\frac{kq_{1}q_{2}}{r^{2}} where k is the Coulomb constant, q_{1} and q_{2} are the charges of the particles, and r is the distance between them.

Muhammed

What is the direction and net electric force on q_{1}= 5µC located at (0,4)r due to charges q_{2}=7mu located at (0,0)m and q_{3}=3\mu C located at (4,0)m?

Kate Reply

what is the change in momentum of a body?

Eunice Reply

what is a capacitor?

Raymond Reply

Capacitor is a separation of opposite charges using an insulator of very small dimension between them. Capacitor is used for allowing an AC (alternating current) to pass while a DC (direct current) is blocked.

Gautam

A motor travelling at 72km/m on sighting a stop sign applying the breaks such that under constant deaccelerate in the meters of 50 metres what is the magnitude of the accelerate

Maria Reply

please solve

Sharon

8m/s²

Aishat

What is Thermodynamics

Muordit

velocity can be 72 km/h in question. 72 km/h=20 m/s, v^2=2.a.x , 20^2=2.a.50, a=4 m/s^2.

Mehmet

A boat travels due east at a speed of 40meter per seconds across a river flowing due south at 30meter per seconds. what is the resultant speed of the boat

Saheed Reply

50 m/s due south east

Someone

which has a higher temperature, 1cup of boiling water or 1teapot of boiling water which can transfer more heat 1cup of boiling water or 1 teapot of boiling water explain your . answer

Ramon Reply

I believe temperature being an intensive property does not change for any amount of boiling water whereas heat being an extensive property changes with amount/size of the system.

Someone

Scratch that

Someone

temperature for any amount of water to boil at ntp is 100⁰C (it is a state function and and intensive property) and it depends both will give same amount of heat because the surface available for heat transfer is greater in case of the kettle as well as the heat stored in it but if you talk.....

Someone

about the amount of heat stored in the system then in that case since the mass of water in the kettle is greater so more energy is required to raise the temperature b/c more molecules of water are present in the kettle

Someone

definitely of physics

Haryormhidey Reply

how many start and codon

Esrael Reply

what is field

Felix Reply

physics, biology and chemistry this is my Field

ALIYU

field is a region of space under the influence of some physical properties

Collete

what is ogarnic chemistry

WISDOM Reply

determine the slope giving that 3y+ 2x-14=0

WISDOM

Another formula for Acceleration

Belty Reply

a=v/t. a=f/m a

IHUMA

innocent

Adah

pratica A on solution of hydro chloric acid,B is a solution containing 0.5000 mole ofsodium chlorid per dm³,put A in the burret and titrate 20.00 or 25.00cm³ portion of B using melting orange as the indicator. record the deside of your burret tabulate the burret reading and calculate the average volume of acid used?

Nassze Reply

how do lnternal energy measures

Esrael

Two bodies attract each other electrically. Do they both have to be charged? Answer the same question if the bodies repel one another.

JALLAH Reply

No. According to Isac Newtons law. this two bodies maybe you and the wall beside you. Attracting depends on the mass och each body and distance between them.

Dlovan

Are you really asking if two bodies have to be charged to be influenced by Coulombs Law?

Robert

like charges repel while unlike charges atttact

Raymond

What is specific heat capacity

Destiny Reply

Specific heat capacity is a measure of the amount of energy required to raise the temperature of a substance by one degree Celsius (or Kelvin). It is measured in Joules per kilogram per degree Celsius (J/kg°C).

AI-Robot

specific heat capacity is the amount of energy needed to raise the temperature of a substance by one degree Celsius or kelvin

ROKEEB

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Source: OpenStax, Precalculus. OpenStax CNX. Jan 19, 2016 Download for free at https://legacy.cnx.org/content/col11667/1.6

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