Problem 13 - College Algebra

Question

For the following exercises, use logarithms to solve. $$ e^{r+10}-10=-42 $$

Step-by-Step Solution

Answer

No real solution, as \(e^{r+10}\) cannot be negative.

Step 1: Simplify the Equation

Start by simplifying the given equation. The equation is \( e^{r+10} - 10 = -42 \). To isolate the exponential term, add 10 to both sides:\[e^{r+10} = -42 + 10\]Which simplifies to:\[e^{r+10} = -32 \]

Step 2: Recognize Inconsistency

Since the base of the natural logarithm \(e\) is a positive number, \(e^{r+10}\) cannot be negative for any real \(r\). Since \( -32\) is negative, there is no real solution to this equation.

Key Concepts

Exponential Functions

Exponential functions are mathematical expressions in which a constant base is raised to a variable exponent. The general form is \[ a^x \] where \( a \) is a positive real number, and \( x \) is the variable exponent. These functions are crucial in describing growth and decay phenomena that occur in many areas like population dynamics and radioactive decay. In particular, the exponential function with base \( e \) (Euler's number, approximately 2.718) is commonly used, especially in calculus and scientific studies.

• The function \( e^x \) grows rapidly as \( x \) increases.
• For negative values of \( x \), \( e^x \) approaches zero but never actually reaches it.
• Exponential growth means that the rate of change of the quantity is proportional to its current value.

Understanding when an exponential equation has no solution or leads to a contradiction, such as in our exercise, is a key skill. For any positive base, like \( e \), raising it to any power will always result in a positive outcome. Thus, caution is necessary when dealing with equations that suggest a negative result for an exponential term.

Real Numbers

Real numbers encompass all the numbers on the number line, including both rational and irrational numbers. This means they include integers, fractions, and decimals that can be positive, negative, or zero. A real number is considered 'real' because it can be represented on this continuous line, unlike imaginary numbers.

• Rational numbers are those that can be expressed as a fraction or ratio, like 1/2 or 3.
• Irrational numbers cannot be expressed exactly as fractions, such as \(\pi\) or \(\sqrt{2}\).
• Every exponential function with a real number as an exponent will result in a real number.

In our example, the failure to find a real solution comes from trying to equalize a positive exponential expression, \( e^{r+10} \), to a negative number \(-32\). Such an outcome is not possible on the real number line.

Natural Logarithm

The natural logarithm, denoted as \( \ln(x) \), is the inverse of the exponential function with base \( e \). It provides the exponent to which \( e \) must be raised to obtain a given number. For example, if \( e^y = x \), then \( \ln(x) = y \). It plays a vital role in solving equations where the variable is in the exponent.

• \( \ln(e) = 1 \), meaning that \( e^1 = e \).
• \( \ln(1) = 0 \) since \( e^0 = 1 \).
• The natural logarithm can only take a positive number as input, indicating that \( \ln(x) \) for \( x \leq 0 \) is undefined.

In solving logarithmic equations, especially those involving the expression \( e^{x} \), we use the natural logarithm to 'undo' the exponentiation. However, as explained in the exercise, if the exponential expression results in a negative, a real logarithmic solution will not exist. Understanding these properties of logarithms is critical in analyzing the logic behind such equations.