Solving Logarithmic Equations: A Step-by-Step Guide

Logarithmic equations are a fundamental part of mathematics, particularly in algebra and calculus. They involve logarithms, which are the inverse of exponential functions. Solving logarithmic equations can seem daunting at first, but with a systematic approach, they become manageable. This article provides a comprehensive guide to understanding and solving logarithmic equations.

Understanding Logarithms

Before diving into solving equations, let’s refresh our understanding of logarithms. A logarithm answers the question: “To what power must we raise a base to get a certain number?” For example, the logarithm of 100 to base 10 is 2, because 10 raised to the power of 2 equals 100.

Mathematically, we express this as:

log_b(x) = y

where:

• b is the base of the logarithm
• x is the number we are taking the logarithm of
• y is the power to which we must raise b to get x

Solving Logarithmic Equations

To solve logarithmic equations, we use the following key properties:

• Property 1: Logarithm of a product: log_b(xy) = log_b(x) + log_b(y)
• Property 2: Logarithm of a quotient: log_b(x/y) = log_b(x) – log_b(y)
• Property 3: Logarithm of a power: log_b(xⁿ) = n * log_b(x)
• Property 4: Logarithmic form and exponential form: If log_b(x) = y, then b^y = x

Steps for Solving Logarithmic Equations

Here’s a step-by-step approach to solving logarithmic equations:

1. Isolate the logarithmic term: If there are multiple terms involving logarithms, try to isolate one of them on one side of the equation.
2. Convert to exponential form: Use the property 4 to rewrite the logarithmic equation in exponential form. This often makes it easier to solve for the unknown variable.
3. Solve for the variable: Apply algebraic techniques to solve for the unknown variable in the exponential equation. Remember to check your solution to ensure it is valid within the domain of the original logarithmic equation.
4. Check for extraneous solutions: Sometimes, solutions obtained algebraically may not be valid in the context of the original logarithmic equation. Always check your solutions by plugging them back into the original equation.

Examples

Let’s illustrate the process with some examples:

Example 1

Solve the equation: log₂(x + 1) = 3

1. The logarithmic term is already isolated.
2. Convert to exponential form: 2³ = x + 1
3. Solve for x: 8 = x + 1, therefore x = 7
4. Check: log₂(7 + 1) = log₂(8) = 3 (The solution is valid)

Example 2

Solve the equation: log₃(x) + log₃(x – 2) = 1

1. Use Property 1 to combine the logarithmic terms: log₃(x(x – 2)) = 1
2. Convert to exponential form: 3¹ = x(x – 2)
3. Solve for x: 3 = x² – 2x, rearrange to get x² – 2x – 3 = 0. Factor the quadratic equation to get (x – 3)(x + 1) = 0. Therefore, x = 3 or x = -1.
4. Check: For x = 3, log₃(3) + log₃(3 – 2) = 1 + 0 = 1 (The solution is valid). For x = -1, log₃(-1) is undefined. Therefore, x = -1 is an extraneous solution.

Conclusion

Solving logarithmic equations is a valuable skill in mathematics. By understanding the properties of logarithms and following a step-by-step approach, you can confidently solve these equations. Remember to always check your solutions to ensure they are valid within the domain of the original equation.