Chapter 9: Problem 54
If \(\log _{b} 3=0.5\) and \(\log _{b} 5=0.7,\) evaluate each expression. See Example 6 $$ \log _{b} 25 $$
Short Answer
Expert verified
The value of \( \log_b 25 \) is 1.4.
Step by step solution
01
Understand the Question
The question asks us to evaluate \( \log_b 25 \) given \( \log_b 3 = 0.5 \) and \( \log_b 5 = 0.7 \). Our goal is to express \( 25 \) in terms of \( 3 \) and \( 5 \), if possible.
02
Identify the Base-Number Relationship
Notice that \( 25 \) can be expressed as \( 5^2 \). This relationship allows us to use the properties of logarithms to simplify \( \log_b 25 \).
03
Apply Logarithm Power Rule
Apply the power rule of logarithms, which states that \( \log_b (a^n) = n \cdot \log_b a \). Hence, \( \log_b (5^2) = 2 \cdot \log_b 5 \).
04
Substitute Given Values
Substitute the given value for \( \log_b 5 \). We have \( 2 \cdot \log_b 5 = 2 \cdot 0.7 = 1.4 \). This simplifies the expression.
05
Conclusion
Thus, the value of \( \log_b 25 \) is 1.4 based on the properties of logarithms and the given values.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Logarithm Properties
Logarithms have a set of essential properties that make them incredibly useful in simplifying and solving mathematical problems. Let's highlight a few key properties:
- Product Property: The logarithm of a product is the sum of the logarithms of the factors, i.e., \( \log_b (xy) = \log_b x + \log_b y \)
- Quotient Property: The logarithm of a quotient is the difference between the logarithms, i.e., \( \log_b \left( \frac{x}{y} \right) = \log_b x - \log_b y \)
- Power Property: The logarithm of a power can be simplified by multiplying the exponent by the logarithm of the base, i.e., \( \log_b (x^n) = n \cdot \log_b x \)
Logarithm Power Rule
The power rule of logarithms is a powerful tool that simplifies the logarithm of a number raised to a power. If you have an expression of the form \( \log_b (a^n) \), this rule lets you take the exponent, \(n\), and multiply it by the logarithm of the base, \(a\). Therefore, \( \log_b (a^n) = n \cdot \log_b a \). This transformation is especially handy when dealing with exponential bases in logarithms.
As seen in the exercise, we can simplify \( \log_b (5^2) \) using the power rule. When applied, this becomes \( 2 \cdot \log_b 5 \). By using substitution with known logarithmic values, it allows complex calculations to become simple arithmetic calculations.
As seen in the exercise, we can simplify \( \log_b (5^2) \) using the power rule. When applied, this becomes \( 2 \cdot \log_b 5 \). By using substitution with known logarithmic values, it allows complex calculations to become simple arithmetic calculations.
Logarithmic Evaluation
Evaluating a logarithm is about finding the exponent to which the base must be raised to get a particular number. For instance, when you evaluate \( \log_b 25 = 1.4 \), you are essentially saying that \( b^{1.4} = 25 \).
The step-by-step solution shows how we use known values (\( \log_b 3 \) and \( \log_b 5 \)) to compute \( \log_b 25 \). By expressing 25 as \( 5^2 \), we apply the power rule to simplify the expression. Finally, substitution with the given \( \log_b 5 = 0.7 \) leads to easy calculation of \(16\). This evaluates \( \log_b 25 \) accurately, showcasing the effectiveness of combining logarithmic properties with specific values.
The step-by-step solution shows how we use known values (\( \log_b 3 \) and \( \log_b 5 \)) to compute \( \log_b 25 \). By expressing 25 as \( 5^2 \), we apply the power rule to simplify the expression. Finally, substitution with the given \( \log_b 5 = 0.7 \) leads to easy calculation of \(16\). This evaluates \( \log_b 25 \) accurately, showcasing the effectiveness of combining logarithmic properties with specific values.
Base Conversion in Logarithms
Converting bases in logarithms is sometimes needed to evaluate logs with non-standard bases. While not required in our specific problem, understanding this technique is valuable for other logarithmic computations. The change of base formula is:
- \( \log_b x = \frac{\log_k x}{\log_k b} \)