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IB DP Maths AA SL Study Notes

1.3.2 Logarithmic Equations

Understanding Logarithms

Logarithms, denoted as log, are the inverse operations to exponentiation. The logarithm logb(a) asks the question: to what power must "b" be raised to produce "a"? Mathematically, if by = a, then logb(a) = y. To grasp the core principles of logarithms, it's beneficial to also understand their relationship with exponential equations.

Properties of Logarithms

  • Product Property: logb(mn) = logb(m) + logb(n)
  • Quotient Property: logb(m/n) = logb(m) - logb(n)
  • Power Property: logb(mn) = n * logb(m)
  • Change of Base Formula: logb(a) = logc(a) / logc(b), where c is a chosen new base.
  • Logarithm of 1: logb(1) = 0 because b0 = 1.
  • Logarithm of the Base: logb(b) = 1 because b1 = b.

Understanding these properties is crucial as they form the foundation for solving logarithmic equations and simplifying logarithmic expressions. Familiarise yourself further with the properties of logarithms to deepen your understanding.

Solving Logarithmic Equations

Method 1: Condensing Logarithms

When an equation has multiple logarithms, use the properties of logarithms to condense them into a single logarithm before solving.

Example 1:

Consider the equation log2(x) + log2(x - 1) = 3.

Using the product property, we combine the logarithms:

log2(x(x - 1)) = 3

Now, convert the logarithmic equation to exponential form:

x(x - 1) = 23

x2 - x - 8 = 0

Factorising the quadratic equation, we find the potential solutions x = 4 and x = -2. However, substituting x = -2 back into the original logarithmic equation, we observe it is not valid since the log of a negative number is undefined. Therefore, x = 4 is the only solution. This method closely relates to understanding the basics of binomial expansion.

Method 2: Using the Change of Base Formula

This method is particularly useful when the bases of the logarithms in the equation are different.

Example 2:

Consider the equation log3(x) = log5(125).

Using the change of base formula, we rewrite the equation:

log10(x) / log10(3) = log10(125) / log10(5)

Now, simplify the equation:

log10(x) / log10(3) = 3

log10(x) = 3 * log10(3)

x = 33

x = 27

When solving equations using the change of base formula, having a strong understanding of domain and range basics is essential.

IB Maths Tutor Tip: Mastering logarithms requires understanding their inverse relationship with exponentiation and applying their properties creatively to simplify and solve equations effectively in diverse mathematical scenarios.

Method 3: Exponentiating to Eliminate Logarithms

In some cases, exponentiating both sides of the equation can eliminate the logarithms, making it easier to solve.

Example 3:

Consider the equation log4(x) = 2.

Exponentiating both sides with base 4, we get:

4(log4(x)) = 42

x = 16

Applications of Logarithmic Equations

Logarithmic equations are prevalent in various scientific and financial calculations, offering a mathematical framework to describe phenomena involving exponential change or growth.

Example 4: pH Calculation

The pH of a solution is calculated using the formula:

pH = -log10([H+])

Where [H+] is the concentration of hydrogen ions in the solution. If a solution has [H+] = 1 x 10(-7) M, find the pH.

pH = -log10(1 x 10(-7))

Using the power property of logarithms:

pH = -(-7 * log10(10))

pH = 7

This application demonstrates how logarithms play a critical role in solving exponential equations in various fields.

Recap and Practice

Logarithmic equations and their properties are pivotal in understanding various mathematical and real-world phenomena. The methods outlined above, including condensing logarithms, using the change of base formula, and exponentiating to eliminate logarithms, are fundamental in solving these equations.

IB Tutor Advice: During revision, practise converting between logarithmic and exponential forms frequently, as it strengthens your ability to interpret and solve logarithmic equations swiftly in exam settings.

Practice Questions

  • Solve the logarithmic equation: log2(x) + log2(x + 2) = 3.
  • Solve for x: log5(x) = log5(3x - 2).
  • If the pH of a solution is 5, find the hydrogen ion concentration [H+].

These practice questions allow you to apply the foundational knowledge you've gained, including properties of logarithms and exponential equations, furthering your understanding of logarithmic equations.

FAQ

Logarithms and exponents are inverse operations, meaning they undo each other. Specifically, if y = bx, then x = logb(y). In other words, if you take the logarithm of a number that is the result of an exponentiation, you get back the original exponent. Similarly, if you exponentiate a number that is the result of a logarithm, you get back the original number. This inverse relationship is fundamental in algebra and calculus, aiding in solving equations, simplifying expressions, and calculating derivatives and integrals, among other applications.

Yes, logarithms are particularly useful for solving equations where the variable is in the exponent, often referred to as exponential equations. By taking the logarithm of both sides of an equation, you can utilise the properties of logarithms to simplify and solve for the variable. For example, in the equation 2x = 8, taking the log base 2 of both sides allows you to bring down the variable from the exponent, resulting in x = log2(8), which simplifies to x = 3, utilising the fact that 23 = 8.

The base of a logarithm significantly impacts its graph. If the base is greater than 1 (e.g., 2, 10), the logarithm function will be increasing and concave down, crossing through the point (1,0) on the graph. If the base is between 0 and 1 (e.g., 1/2), the logarithm function will be decreasing and concave up, but still passing through (1,0). The base determines the steepness of the graph; larger bases result in steeper graphs, while smaller bases produce more gradual slopes. Understanding the impact of the base on the graph helps in sketching and interpreting logarithmic functions in various mathematical and real-world contexts.

Logarithms are widely used in various scientific domains due to their ability to manage and simplify calculations involving exponential growth or decay, and multiplicative processes. For instance, in biology, the pH scale, which measures acidity, is based on a logarithmic scale. In astronomy, the magnitude scale, which quantifies the brightness of celestial objects, is logarithmic. In seismology, the Richter scale, which gauges the energy released by earthquakes, employs logarithms to handle the vast range of energy release. These logarithmic scales enable scientists to easily compare and analyse quantities that can vary over a large range of magnitudes.

Logarithms of negative numbers and zero are undefined in the real number system. The reason behind this lies in the definition of logarithms: if by = a, then logb(a) = y. If you consider a to be negative or zero, there is no real number y that you can raise b to in order to get a negative number or zero, assuming b is a positive real number. This is because any positive number to the power of any real number is always positive. Thus, logarithms of negative numbers and zero do not exist in the realm of real numbers.

Practice Questions

Solve the logarithmic equation log_2(x) + log_2(x - 7) = 3.

To solve the logarithmic equation log2(x) + log2(x - 7) = 3, we can utilise the properties of logarithms. Firstly, we can combine the two logarithms on the left-hand side using the product property of logarithms, which states that logb(m) + logb(n) = logb(mn). Therefore, we get log2(x(x - 7)) = 3. Next, we can rewrite the equation in exponential form to eliminate the logarithm: x(x - 7) = 23. Simplifying, we get x2 - 7x - 8 = 0. Factoring the quadratic equation, we find (x - 8)(x + 1) = 0, thus x = 8 or x = -1. However, substituting x = -1 back into the original logarithmic equation, we find it is not valid since the log of a negative number is undefined. Therefore, x = 8 is the only solution.

Solve the logarithmic equation 2log_3(x) - log_3(5) = 3.

To solve the logarithmic equation 2log3(x) - log3(5) = 3, we can again utilise the properties of logarithms. Firstly, we can simplify the equation using the power property of logarithms, which states that a logb(c) = logb(ca), to get log3(x2) - log3(5) = 3. Next, we can use the quotient property of logarithms, which states that logb(m) - logb(n) = logb(m/n), to combine the two logarithms on the left-hand side: log3(x2/5) = 3. Now, we can rewrite the equation in exponential form to eliminate the logarithm: x2/5 = 33. Simplifying, we get x2 = 135. Taking the square root of both sides, we find x = √135 or x = -√135. However, substituting x = -√135 back into the original logarithmic equation, we find it is not valid since the log of a negative number is undefined. Therefore, x = √135 is the only solution.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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