Problem 20 - College Algebra and Calculus: An Applied Approach

Question

Write the logarithm in terms of natural logarithms.\(\log _{1 / 3} x\)

Step-by-Step Solution

Answer

Therefore, the expression \(\log _{1 / 3} x\) in terms of natural logarithms becomes \( -\frac{\ln x}{\ln 3} \).

Step 1: Apply the Change of Base Formula

Replace the base 1/3 logarithm with the change of Base formula. This leads to \(\frac{\ln x}{\ln(1 / 3)}\).

Step 2: Simplify the Denominator

Recognize that \( \ln(1 / 3) \) is the same as \( -\ln 3 \) since the natural logarithm of a reciprocal is the negative of the natural logarithm. Hence, the expression simplifies to \( -\frac{\ln x}{\ln 3} \).

Key Concepts

Understanding Natural Logarithms

Logarithms are a cornerstone in the landscape of mathematics, often used to solve problems involving exponential relationships. Among the various types of logarithms, natural logarithms are particularly important.

A natural logarithm is a logarithm with the base 'e', where 'e' is an irrational constant approximately equal to 2.71828. When you see the notation \(\ln x\), it represents the power to which 'e' must be raised to obtain the value 'x'. For example, if \(\ln x = 3\), this means that \(e^3 = x\). This is a crucial concept since natural logarithms are extensively used in various fields such as science, engineering, and economics due to their natural occurrence in growth processes and compound interest calculations.

Simplifying Expressions with Natural Logarithms

Simplifying logarithmic expressions using the natural log can make solving equations more manageable. For instance, using properties of logarithms, such as the fact that \(\ln(ab) = \ln a + \ln b\) or \(\ln(a^b) = b\ln a\), can help break down complex expressions into simpler terms that can be more easily manipulated and understood.

Change of Base Formula

When dealing with logarithms, you are not always limited to working with a base 'e' or base 10. There might be occasions when you need to convert logarithms from one base to another, and this is where the change of base formula comes into play.

The formula states that for any positive numbers 'a', 'b', and 'c', where 'a' is not equal to 1, the logarithm of 'c' with base 'a' can be expressed as: \(\log_a c = \frac{\ln c}{\ln a}\) or alternatively \(\log_a c = \frac{\log_b c}{\log_b a}\), if you prefer a base 'b' other than 'e'. This powerful tool allows you to evaluate logarithms on a calculator that may only have keys for base 10 or base 'e' logarithms, hence expanding the reach and ease of computations involving logarithms.

Applying the Change of Base Formula

In the example provided, the change of base formula has been used to express \(\log_{1/3} x\) in terms of natural logarithms as \(\frac{\ln x}{\ln(1/3)}\). It's a simple yet effective method that facilitates the evaluation of logarithms with unusual bases, making them easier to manipulate and solve.

Simplifying Logarithms

Simplifying logarithmic expressions is a useful skill that can make solving mathematical problems more straightforward. When simplifying logs, there are several properties and rules that can be applied. Some of these include the product rule, the quotient rule, and the power rule, all of which help in breaking down complex logarithmic expressions.

A valuable step in the process of simplification is recognizing and utilizing the relationships between logs and exponentials. For instance, an expression like \(\ln(1 / 3)\) may seem intimidating, but understanding that the natural logarithm of a fraction is the negative of the logarithm of its reciprocal – which stems from the property \(\ln(1/x) = -\ln(x)\) – can significantly simplify the expression to \(\ln(1 / 3) = -\ln 3\).

The ability to simplify logarithms not only allows us to solve equations more efficiently but also to understand the relationship between numbers and their logarithms better, ultimately leading to a deeper appreciation of the logarithmic function's practicality and versatility in mathematics.