Problem 27 - Contemporary Precalculus

Question

Evaluate the given expression without using a calculator. $$\ln e^{15}$$

Step-by-Step Solution

Answer

Answer: The value of the expression $\ln e^{15}$ is 15.

Step 1: Recall the properties of ln and the exponential function e^x

Recall that the natural logarithm function, ln, is the inverse of the exponential function with base e. This means that if we apply ln to an expression involving e raised to a power, the natural logarithm "cancels out" the exponential function, leaving us with the exponent: $$\ln(e^x) = x$$

Step 2: Apply the inverse property to the given expression

Now, we can apply the inverse property to the given expression: $$\ln e^{15}$$ Since ln "cancels out" the exponential function with base e, we are left with the exponent, 15: $$\ln e^{15} = 15$$ So, the value of the expression is 15.

Key Concepts

Inverse Functions

Inverse functions are mathematical functions that essentially reverse the effect of their counterparts. When you apply an inverse function to the result of its corresponding function, you will end up back where you started. Consider two functions, \(f(x)\) and \(g(x)\). If applying \(g(x)\) to the result of \(f(x)\) brings you back to \(x\), then \(g(x)\) is said to be the inverse of \(f(x)\), shown as \(g(f(x)) = x\).

The relationship between exponential functions and logarithms is a classic example of inverse functions. Specifically, the natural logarithm, \(\ln\), is the inverse function of the exponential function with base \(e\), which means:

• \(\ln(e^x) = x\)
• \(e^{\ln(x)} = x\)

This inverse relationship is key to solving problems that involve combined exponential and logarithmic expressions without computational tools.

Properties of Logarithms

Logarithms possess several key properties that make them vital tools for simplifying and solving mathematical problems. Here are some important properties:

• Product Property: \(\ln(ab) = \ln(a) + \ln(b)\)
• Quotient Property: \(\ln\left(\frac{a}{b}\right) = \ln(a) - \ln(b)\)
• Power Property: \(\ln(a^b) = b \cdot \ln(a)\)
• Inverse Property: \(\ln(e^x) = x\)

The inverse property is especially crucial, as it allows us to simplify expressions involving the natural logarithm and the exponential function. For example, \(\ln(e^{15}) = 15\) because the \(\ln\) function removes the exponential function with base \(e\), leaving us directly with the exponent. Understanding these properties can help simplify seemingly complex expressions.

Exponential Functions

Exponential functions are powerful mathematical constructs that can be identified by their constant base and variable exponent. They take the form \(y = a^x\), where \(a\) is a constant and \(x\) is the variable exponent.

The exponential function with base \(e\), denoted as \(e^x\), is more than just a standard example; it is the foundation of the natural logarithm. The number \(e\) is approximately equal to 2.71828 and is unique because of its natural occurrence in calculus, complex numbers, and real-world phenomena such as growth and decay processes.

• Exponential functions grow very rapidly for positive values of \(x\).
• When \(x\) is negative, the value of the function approaches zero.
• They are continuous and smooth, without any breaks or gaps over their domain.

Understanding exponential functions, especially those with base \(e\), is essential for mastering the concept of natural logarithms since they are inverses. Recognizing how these functions interact can enable problem-solving involving expressions where both functions are present.