Problem 11 - Contemporary Precalculus

Question

In Exercises \(11-16,\) let \(u=\ln x\) and \(v=\ln y .\) Write the given expression in terms of u and v. For example, $$\ln x^{3} y=\ln x^{3}+\ln y=3 \ln x+\ln y=3 u+v$$ $$\ln \left(x^{2} y^{5}\right)$$

Step-by-Step Solution

Answer

Question: Express the given expression ln(x^2 y^5) in terms of u = ln(x) and v = ln(y) using the natural logarithm properties. Answer: ln(x^2 y^5) can be written in terms of u and v as 2 * u + 5 * v.

Step 1: Applying the first logarithm property

Apply the first logarithm property ln(a * b) = ln(a) + ln(b) to the given expression ln(x^2 * y^5). So, we have ln(x^2 y^5) = ln(x^2) + ln(y^5)

Step 2: Applying the second logarithm property

Apply the second logarithm property ln(a^k) = k * ln(a) to both terms in the result from Step 1. So, we get 2 * ln(x) + 5 * ln(y)

Step 3: Substituting u and v

Substitute u = ln(x) and v = ln(y) in the expression obtained in Step 2. So, we have 2 * u + 5 * v The given expression ln(x^2 y^5) can be written in terms of u and v as: 2 * u + 5 * v

Key Concepts

Natural Logarithm

The natural logarithm is a fundamental mathematical function, denoted as \( \ln(x) \), and it refers to the logarithm to the base \( e \), where \( e \) is the Euler's constant, approximately equal to 2.71828. In simple terms, if \( \ln(x) = y \), then we can understand it as \( e^y = x \). This special logarithm is widely used because of its natural properties in growth and decay processes, such as interest in finance or populations in biology.

Students sometimes struggle with understanding why we use \( e \), but it's because its rate of growth is considered to be the natural rate of increase. For every unit increase in value, you can think of e as the factor by which a quantity multiplies itself. The natural logarithm, therefore, helps us find the time or the rate at which these changes happen.

The ease of differentiating and integrating natural logarithms makes them especially useful in calculus. In the context of the textbook exercise, understanding the natural logarithm is key to operating with logarithmic expressions and employing algebraic manipulation.

Logarithmic Expressions

Working with logarithmic expressions often requires understanding their unique properties and rules. These are tools that help simplify complex logarithms into more manageable parts, or sometimes even single variables. For instance, the first property used in the exercise, \( \ln(a * b) = \ln(a) + \ln(b) \), shows how a product within a logarithm can be broken down into a sum of logarithms.

Key Properties of Logarithms

• Product Property: \( \ln(a * b) = \ln(a) + \ln(b) \)
• Quotient Property: \( \ln(\frac{a}{b}) = \ln(a) - \ln(b) \)
• Power Property: \( \ln(a^k) = k * \ln(a) \)

The exercise demonstrates the use of the power property to separate the exponent from the term inside the logarithm. It's also important to note the base of the logs being used. For the natural logarithm, the base is \( e \), but these properties hold true for logs of any base. When students grasp these properties, they can manipulate logarithmic expressions to simplify or solve them.

Using these properties strategically can transform a dense logarithmic problem into a straightforward algebraic one.

Algebraic Manipulation

Performing algebraic manipulation involves rearranging, simplifying, or transforming algebraic expressions using a variety of techniques and properties of algebra. These techniques can include basic operations such as addition, subtraction, multiplication, and division; distributing and factoring; and using properties of exponents and logarithms.

Applying Algebra to Logarithms

Our exercise is a prime example of using algebraic manipulation to simplify logarithmic expressions. After applying logarithm properties, we use substitution—replacing \( \ln(x) \) with \( u \) and \( \ln(y) \) with \( v \)—to turn a complex expression into a much simpler linear combination of variables: \( 2u + 5v \).

This technique is particularly useful in higher-level mathematics, where simplifying an expression can make problems more tractable. In algebraic manipulation, the goal is often to reduce the problem to a form that is easier to interpret or solve, as shown in the step-by-step solution. Remember, the more practice you get with these manipulations, the better you'll understand how to apply them across a variety of mathematical problems.