Question

Use the properties of logarithms to simplify the following functions before computing \(f^{\prime}(x)\). $$f(x)=\ln (3 x+1)^{4}$$

Short answer

Answer: The simplified version of the function is \(f(x) = 4\,\ln(3x+1)\) and its derivative is \(f'(x) = \frac{12}{3x+1}\).

Step-by-step solution

Simplify the function using the properties of logarithms.

Use the property \(\ln(a^b) = b\, \ln(a)\): $$f(x) = \ln(3x+1)^4 = 4\,\ln(3x+1)$$ Now that we have a simplified expression for \(f(x)\), we can compute \(f'(x)\).

Compute the function's derivative

Use the chain rule to compute the derivative of the composite function: \((g(h(x)))' = g'(h(x))\,h'(x)\). Here, let \(g(x) = 4\,\ln(x)\) and \(h(x) = 3x+1\). Then, \(g'(x)\) and \(h'(x)\) are: $$g'(x) = \frac{4}{x} \quad\text{and}\quad h'(x) = 3$$ We can now apply the chain rule: $$f'(x) = 4\,\frac{1}{3x+1}\cdot 3 = \frac{12}{3x+1}$$ So, after using the properties of logarithms and applying the chain rule, the simplified function and its derivative are: $$f(x) = 4\,\ln(3x+1) \quad\text{and}\quad f'(x) = \frac{12}{3x+1}$$

Key concepts

Logarithmic Differentiation

Logarithmic differentiation is a powerful mathematical technique which simplifies the process of finding derivatives, especially when dealing with products, quotients, and powers of functions. It exploits the properties of logarithms to turn multiplication into addition, division into subtraction, and powers into products. This is particularly useful when dealing with complicated expressions that would be difficult to differentiate using basic rules.

In the given exercise, the function presented is \( f(x) = \ln(3x+1)^4 \). Using the property of logarithms that allows us to bring down the exponent (\(\ln(a^b) = b \, \ln(a)\)), we simplify the expression to \(4\ln(3x+1)\), which is easier to handle when taking the derivative. This initial step is the essence of logarithmic differentiation: simplify first, calculate the derivative second.

Once the expression is simplified, we move on to apply the differentiation rules, confident that the mathematical 'heavy lifting' has been managed through our clever use of logarithmic properties. This makes calculating derivatives more manageable and streamlined, and it's a testament to logarithmic differentiation's utility in calculus.

Chain Rule in Calculus

The chain rule is among the most important rules in calculus for finding derivatives of composite functions. When we have a function composed of one function inside another, we can't differentiate each part separately. Instead, we use the chain rule, which provides a method to differentiate the entire composite function systematically.

The chain rule states that the derivative of a composite function is the derivative of the outer function evaluated at the inner function times the derivative of the inner function. Mathematically, this is represented as \( (g(h(x)))' = g'(h(x)) \, h'(x) \).

In our exercise, \( g(x) = 4\ln(x) \) and \( h(x) = 3x+1 \). By finding \( g'(x) = \frac{4}{x} \) and \( h'(x) = 3 \) first, we can then use the chain rule to effectively find \( f'(x) \) as \(4 \frac{1}{3x+1} \cdot 3\), or simplified to \( \frac{12}{3x+1} \).

Utilizing the chain rule in this manner allows students to tackle more complex derivatives methodically, breaking down the problem into manageable steps and ultimately revealing the simplicity within the complexity.

Derivative of Logarithmic Functions

Logarithmic functions pose their own unique challenges in calculus, but once their derivative principles are understood, they can be approached with confidence. The derivative of the natural logarithm function, \(\ln(x)\), is one of the fundamental derivatives in calculus and is equal to \( \frac{1}{x} \).

However, when working with logarithmic functions that include additional components, such as coefficients or composite functions, the differentiation process becomes slightly more involved. For example, if the function is a multiplied constant times the natural logarithm, as in our example \( f(x) = 4\ln(3x+1) \), the constant can be treated separately from the \(\ln \) function.

To find \(f'(x)\) of our function, we apply the derivative of the natural logarithm to the inner function \(3x+1\), multiplying it by the derivative of the inner function itself, which is 3. The constant 4 simply remains as a constant factor in the derivative, giving us the final result of \( \frac{12}{3x+1} \). This consistent approach to logarithmic differentiation fluently combines the understanding of logarithms with the mechanics of differentiation to effectively deal with a variety of functions.