Problem 36 - Precalculus with Limits

Question

Using Properties of Logarithms In Exercises \(21-36\) , find the exact value of the logarithmic expression without using a calculator. (If this is not possible, then state the reason.) $$\log _{4} 2+\log _{4} 32$$

Step-by-Step Solution

Answer

The exact value of the logarithmic expression \(\log _{4} 2+ \log _{4} 32\) is 3.

Step 1: Apply the product property of logarithms

The product property of logarithms states that the sum of the logs equals the log of the product. Thus, we can combine \(\log _{4} 2+ \log _{4} 32\) to become \(\log _{4} (2 \times 32)\).

Step 2: Simplify the product

Multiply 2 by 32 to get 64. Substituting this back into the equation gives us \(\log _{4} 64\).

Step 3: Calculate the log base 4 of 64

Using the special log property if \(a^m = b\), then \(\log_a b = m\), we find that \(4^3 = 64\). Consequently, \(\log _{4} 64 = 3\).

Key Concepts

Logarithmic Expressions

Logarithmic expressions represent the power to which a number, called the base, must be raised to produce a given number. Understanding these expressions is crucial for solving various mathematical problems, especially in algebra and calculus. To comprehend a logarithmic expression like \( \log_b a \), you should know that it asks the question: 'To what exponent must the base \( b \) be raised to get \( a \) as the result?'.

Take for example \( \log_4 2 \). This expression seeks to find the exponent that makes 4 become 2, which is not immediately obvious since 2 is not a power of 4. However, by using properties of logarithms, it can still be calculated. If the base and the number are the same, the answer is always 1, because any number raised to the power of 1 is itself. On the other hand, if the number is a clear power of the base, like \( 4^3 = 64 \), then \( \log_4 64 \) simply is 3. But not all logarithmic expressions result in integers, and we may end up with fractions or irrational numbers.

Product Property of Logarithms

The product property of logarithms is a powerful tool that simplifies complex logarithmic calculations. It states that the log of a product is the sum of the logs: \(\log_b (mn) = \log_b m + \log_b n \). This allows for the breakdown of tricky logarithmic terms into more manageable pieces.

In practice, when you encounter a logarithmic expression such as \(\log_4 2 + \log_4 32 \), you can use this property to combine them into a single logarithm, \( \log_4 (2 \times 32) \), hence reducing the problem into finding the logarithm of a single number. This property not only simplifies the calculations but also sometimes allows for solving otherwise unsolvable problems by hand. When the numbers inside the log function are not perfect powers of the base, like the example \( 2 \times 32 \), this property becomes particularly valuable. Using this property encourages a more straightforward approach to the calculation, by focusing on a single logarithmic value.

Logarithm Calculation without a Calculator

It can seem daunting to calculate logarithms without a calculator, but understanding certain base principles might simplify the process. Notably, the relationship \( a^m = b \Rightarrow \log_a b = m \) is the most direct way to find logarithms of numbers that are whole powers of the base.

Here's how you can apply this to calculate \(\log_4 64 \) without a calculator:

• First, recognize that 64 is a power of 4: \(4^3 = 64 \).
• Now apply the relationship: Since \( 4^3 = 64 \), it means \( \log_4 64 = 3 \).

This process requires familiarity with powers of numbers and, sometimes, might involve a bit of trial and error or estimation. After all, logarithms are essentially exponents, so getting comfortable with exponential relations is key. To get better at manual logarithm calculations, practice with powers of various numbers to realize how they relate to their logarithms.