Logarithmic Function Identity: Functions and Their Modeling - Mathematics (Grade 11)

Understanding Logarithmic Identities

Logarithmic identities are special properties that apply to all logarithmic functions. These properties are very helpful in simplifying calculations and solving complex logarithmic equations.

Before discussing logarithmic identities, let's recall that logarithms are the inverse of exponents. If $b^x = a$ , then $^b\log a = x$ .

Basic Logarithmic Identities

Product Identity

^b\log(MN) = {^b\log M} + {^b\log N}

The logarithm of a product equals the sum of the logarithms of each number.

Example:

^2\log(8 \times 4) = {^2\log 8} + {^2\log 4} = 3 + 2 = 5

Quotient Identity

^b\log\left(\frac{M}{N}\right) = {^b\log M} - {^b\log N}

The logarithm of a quotient equals the difference between the logarithm of the numerator and the logarithm of the denominator.

Example:

^3\log\left(\frac{81}{9}\right) = {^3\log 81} - {^3\log 9} = 4 - 2 = 2

Power Identity

^b\log(M^p) = p \cdot {^b\log M}

The logarithm of a number raised to a power equals the power multiplied by the logarithm of that number.

Example:

^2\log(4^3) = 3 \cdot {^2\log 4} = 3 \times 2 = 6

Special Logarithmic Identities

Change of Base

^b\log M = \frac{^a\log M}{^a\log b}

This identity allows us to change the logarithm base as needed.

Example:

^3\log 9 = \frac{^{10}\log 9}{^{10}\log 3} = \frac{0.954}{0.477} = 2

Equality Identity

If $^b\log M = {^b\log N}$ , then $M = N$

Two numbers that have the same logarithmic value (with the same base) must be the same number.

Inequality Identity

If $b > 1$ and $^b\log M < {^b\log N}$ , then $M < N$
If $0 < b < 1$ and $^b\log M < {^b\log N}$ , then $M > N$

Applications of Logarithmic Identities

Simplifying Expressions

Simplify $^2\log 32 + {^2\log 8} - {^2\log 4}$

Solution:

^2\log 32 + {^2\log 8} - {^2\log 4}

Solving Equations

Find the value of $x$ if $^3\log x + {^3\log 9} = {^3\log 81}$

Solution:

^3\log x + {^3\log 9} = {^3\log 81}

Example: At any given time, the price of a used car is not proportional to its current price. If a new car costs $200 \text{ million rupiah}$ and after $5 \text{ years}$ becomes $100 \text{ million rupiah}$ , determine the car's price after $10 \text{ years}$ of use.

Solution:

H(0) = 200 \text{ million, so} 200 = ce^0 = c

From these results, the car's price at any time $t$ is:

H(t) = 200e^{-0.1386t}

Therefore, the car's price after $10 \text{ years}$ of use is:

H(10) = 200e^{-0.1386(10)} = 200e^{-1.386} \approx 50 \text{ million rupiah}

Exercises

Problem $1$

Simplify: $^5\log 125 + {^5\log 25} - {^5\log 5}$

Problem $2$

If $^2\log x = 3$ and $^2\log y = 5$ , find the value of $^2\log(xy)$

Problem $3$

Find the value of $x$ if $^4\log(x-1) = {^4\log 16} - {^4\log 2}$

Answer Key

Answer $1$

^5\log 125 + {^5\log 25} - {^5\log 5}

Answer $2$

^2\log(xy) = {^2\log x} + {^2\log y}

Answer $3$

^4\log(x-1) = {^4\log 16} - {^4\log 2}

Understanding Logarithmic Identities

Logarithmic identities are special properties that apply to all logarithmic functions. These properties are very helpful in simplifying calculations and solving complex logarithmic equations.

Before discussing logarithmic identities, let's recall that logarithms are the inverse of exponents. If $b^x = a$ , then $^b\log a = x$ .

Basic Logarithmic Identities

Product Identity

^b\log(MN) = {^b\log M} + {^b\log N}

The logarithm of a product equals the sum of the logarithms of each number.

Example:

^2\log(8 \times 4) = {^2\log 8} + {^2\log 4} = 3 + 2 = 5

Quotient Identity

^b\log\left(\frac{M}{N}\right) = {^b\log M} - {^b\log N}

The logarithm of a quotient equals the difference between the logarithm of the numerator and the logarithm of the denominator.

Example:

^3\log\left(\frac{81}{9}\right) = {^3\log 81} - {^3\log 9} = 4 - 2 = 2

Power Identity

^b\log(M^p) = p \cdot {^b\log M}

The logarithm of a number raised to a power equals the power multiplied by the logarithm of that number.

Example:

^2\log(4^3) = 3 \cdot {^2\log 4} = 3 \times 2 = 6

Special Logarithmic Identities

Change of Base

^b\log M = \frac{^a\log M}{^a\log b}

This identity allows us to change the logarithm base as needed.

Example:

^3\log 9 = \frac{^{10}\log 9}{^{10}\log 3} = \frac{0.954}{0.477} = 2

Equality Identity

If $^b\log M = {^b\log N}$ , then $M = N$

Two numbers that have the same logarithmic value (with the same base) must be the same number.

Inequality Identity

If $b > 1$ and $^b\log M < {^b\log N}$ , then $M < N$
If $0 < b < 1$ and $^b\log M < {^b\log N}$ , then $M > N$

Applications of Logarithmic Identities

Simplifying Expressions

Simplify $^2\log 32 + {^2\log 8} - {^2\log 4}$

Solution:

^2\log 32 + {^2\log 8} - {^2\log 4}

Solving Equations

Find the value of $x$ if $^3\log x + {^3\log 9} = {^3\log 81}$

Solution:

^3\log x + {^3\log 9} = {^3\log 81}

Real-Life Applications

Richter Scale

Earthquake strength is measured using the Richter scale which is based on logarithms:

R = \log\left(\frac{I}{I_0}\right)

Where:

$R$ = Richter scale value
$I$ = earthquake intensity
$I_0$ = reference intensity (zero level)

Example: An earthquake that occurred in Haiti in $2010$ had an intensity of $10^7\text{ times}$ compared to zero-level earthquakes. What is the Richter scale strength of that earthquake?

Solution:

R = \log\left(\frac{I}{I_0}\right)

Therefore, the earthquake in Haiti in $2010$ had a strength of $7$ on the Richter scale.

Battery Charging

Battery charging time can be calculated using the logarithmic formula:

t = -\frac{1}{k}\ln\left(1 - \frac{C}{C_0}\right)

Where:

$t$ = charging time (in minutes)
$k$ = charging constant
$C$ = desired capacity
$C_0$ = maximum capacity

Example: Determine the time required to charge a battery from empty to $90\%$ full. Assume $k = 0.02$ .

Solution:

t = -\frac{1}{k}\ln\left(1 - \frac{C}{C_0}\right)

Therefore, the charging time is approximately $115 \text{ minutes}$ .

Car Price Depreciation

Logarithmic functions are also used for modeling decay/depreciation with the formula:

H(t) = ce^{kt}

where $H(t)$ is the value at time $t$ .

Solution:

H(0) = 200 \text{ million, so} 200 = ce^0 = c

From these results, the car's price at any time $t$ is:

H(t) = 200e^{-0.1386t}

Therefore, the car's price after $10 \text{ years}$ of use is:

H(10) = 200e^{-0.1386(10)} = 200e^{-1.386} \approx 50 \text{ million rupiah}

Exercises

Problem $1$

Simplify: $^5\log 125 + {^5\log 25} - {^5\log 5}$

Problem $2$

If $^2\log x = 3$ and $^2\log y = 5$ , find the value of $^2\log(xy)$

Problem $3$

Find the value of $x$ if $^4\log(x-1) = {^4\log 16} - {^4\log 2}$

Answer Key

Answer $1$

^5\log 125 + {^5\log 25} - {^5\log 5}

Answer $2$

^2\log(xy) = {^2\log x} + {^2\log y}

Answer $3$

^4\log(x-1) = {^4\log 16} - {^4\log 2}