Problem 60 - College Algebra Essentials

Question

Use properties of logarithms to condense logarithmic expression. Write the expression as a single logarithm whose coefficient is 1. Where possible, evaluate logarithmic expressions without using a calculator. \(8 \ln (x+9)-4 \ln x\)

Step-by-Step Solution

Answer

The condensed form of the given logarithmic expression is \(\ln ((x+9)^8 / x^4)\). This doesn't simplify to a numeric answer without a calculator, as it depends on the value of x.

Step 1: Apply the Power Rule of Logarithms

The power rule of logarithms states that the logarithm of a number raised to a power is equal to the product of that power and the logarithm of the number itself. Here, apply the power rule to the given expression: \(8 \ln (x+9) - 4 \ln x\) becomes \(\ln ((x+9)^8) - \ln (x^4)\).

Step 2: Apply the Quotient Rule of Logarithms

The quotient rule of logarithms states that the logarithm of the ratio of two numbers is equivalent to the difference of their individual logarithms. Apply this rule to the expression from Step 1: \(\ln ((x+9)^8) - \ln (x^4)\) becomes \(\ln ((x+9)^8 / x^4)\).

Step 3: Simplify

At this stage, we can further simplify the expression \(\ln ((x+9)^8 / x^4)\), as it does not evaluate to a single number without a calculator.

Key Concepts

Understanding the Logarithm Power Rule

When diving into logarithms, the power rule stands out as a pivotal concept for simplifying logarithmic expressions. This powerful tool states that for any positive number 'a', not equal to 1, and any real numbers 'b' and 'c', the rule looks like: \[\begin{equation} \log_a(b^c) = c \cdot \log_a(b) \end{equation}\]By applying this rule, you can seamlessly move the exponent in a log expression out in front, turning multiplication into a more manageable addition problem. This technique is particularly useful when you’re faced with the task of condensing multiple logarithms into a single expression. For example:

• If we have an expression like \(8 \ln(x+9)\), we can rewrite it as \(\ln((x+9)^8)\).

This process makes the rest of the simplification much easier, laying the groundwork for further manipulation of the logarithmic expression.

Applying the Logarithm Quotient Rule

Following the power rule, the quotient rule is another essential aspect of working with logarithms. The quotient rule is all about division inside a logarithmic expression. It tells us that the logarithm of a quotient – that is, one expression divided by another – can be expressed as the subtraction of two logarithms. Mathematically, the rule is expressed as:

\[\begin{equation}\log_a\left(\frac{b}{c}\right) = \log_a(b) - \log_a(c)\end{equation}\]In a practical sense, this rule allows you to break down complex expressions into simpler, more digestible parts.

For instance:

• When confronted with an expression like \(\ln((x+9)^8) - \ln(x^4)\), it simplifies to \(\ln\left(\frac{(x+9)^8}{x^4}\right)\), transforming a potentially challenging subtraction problem into a single, elegant logarithmic operation.

Through the quotient rule, we see that division inside a log reduces to straightforward subtraction outside the log, a transformation that tremendously aids in the simplification process.

Simplifying Logarithmic Expressions

Finally, the ultimate goal when working with logarithmic expressions is often to simplify them. Simplification makes complex concepts easier to grasp and calculations more manageable. This endeavor often involves utilizing the power and quotient rules, as well as other properties of logarithms, to break down and reassemble expressions into their most basic form. This process does not only assist in finding the final solution but also aids in understanding the deeper relationships between the numbers and operations involved.

With the expression \(\ln \left(\frac{(x+9)^8}{x^4}\right)\), you are now at a stage where further simplifications might be possible, depending on the specific characteristics of 'x'. However, without using a calculator, the expression remains as is, representing a condensed form that's easy to work with for future algebraic operations. By reaching this point, you have a single logarithmic term that subtly encodes the history of the operations that produced it.