Question

Calculate the derivative of the following functions. $$y=4 \log _{3}\left(x^{2}-1\right)$$

Short answer

Answer: The derivative of the function is $$\frac{dy}{dx} = \frac{8x}{(x^2 - 1) \ln 3}$$.

Step-by-step solution

Rewrite using the natural logarithm

Recall that an arbitrary logarithm can be rewritten using the natural logarithm. The base change formula will be helpful here: $$y = 4 \log_{3}\left(x^2 - 1\right) = \frac{4}{\ln 3} \ln\left(x^2 - 1\right)$$

Differentiate using the chain rule

Now apply the chain rule, which states that the derivative of a composite function is the derivative of the outer function times the derivative of the inner function. So, we have to first differentiate the natural logarithm and then differentiate the quadratic function. The chain rule can be written as: $$\frac{dy}{dx} = \frac{d(\frac{4}{\ln 3} \ln (x^2 -1))}{dx} = \frac{4}{\ln 3} \cdot \frac{d\ln(x^2 - 1)}{dx}$$

Differentiate the natural logarithm function

Differentiate the natural logarithm function with the chain rule, and apply the power rule to the inner function: $$\frac{d\ln(x^2-1)}{dx} = \frac{1}{x^2 - 1} \cdot \frac{d(x^2 - 1)}{dx}$$

Differentiate the quadratic function

Now differentiate the inner quadratic function: $$\frac{d(x^2 - 1)}{dx} = 2x$$

Combine the results

Combine the results from Steps 2, 3, and 4: $$\frac{dy}{dx} = \frac{4}{\ln 3} \cdot \frac{1}{x^2 - 1} \cdot 2x$$

Simplify the derivative

Finally, simplify the expression by multiplying through: $$\frac{dy}{dx} = \frac{8x}{(x^2 - 1) \ln 3}$$ So, the derivative of the function is: $$\frac{dy}{dx} = \frac{8x}{(x^2 - 1) \ln 3}$$

Key concepts

Chain Rule

Understanding the chain rule is crucial when differentiating composite functions. Picture it as a process of peeling an onion, layer by layer. For a composite function, where one function is nested within another, the chain rule states that you differentiate the outer function and multiply it by the derivative of the inner function.

For example, if you're faced with a function like \(\ln (x^2 - 1)\), the outer function is the logarithm and the inner function is \(x^2 - 1\). To differentiate this using the chain rule, you'd take the derivative of \(\ln u\) where \(u = x^2 - 1\), and multiply it by the derivative of \(u\) with respect to \(x\). This step is essential for succeeding in calculus, especially when dealing with complex expressions.

Natural Logarithm

The natural logarithm, denoted as \(\ln\), is a special logarithm with the base \(e\), where \(e\) is an irrational and transcendental number approximately equal to 2.71828. It emerges naturally in calculus because of its unique properties, particularly when dealing with growth processes and areas under hyperbola.

When you see a logarithmic function in an expression you're meant to differentiate, understanding that the derivative of \(\ln x\) with respect to \(x\) is \(1/x\) is pivotal. This knowledge simplifies the process significantly, positioning you to tackle a wide variety of calculus problems with confidence.

Base Change Formula

In calculus, the base change formula is a helpful tool that allows you to rewrite logarithms of any base in terms of natural logarithms. The formula is \(\log_a(b) = \frac{\ln(b)}{\ln(a)}\), where \(a\) is the base of the logarithm and \(b\) is the value being logged.

When faced with different bases, the base change formula becomes invaluable. For instance, in our exercise, by using this formula, we convert \(\log_3(x^2 - 1)\) to a form that involves the natural logarithm, making it much more manageable to take the derivative using rules that are typically easier to apply to natural logarithms.

Power Rule

The power rule is a basic differentiation rule used to handle functions of the form \(x^n\). It states that if you have a power function, its derivative is \(n * x^{n - 1}\). So, if you need to differentiate \(x^2\), the power rule tells you to bring down the exponent as a coefficient and subtract one from the exponent.

In the given exercise, the power rule enables you to take the derivative of the inner function \(x^2 - 1\), producing \(2x\). It’s a straightforward yet powerful tool at your disposal, especially when dealing with polynomial functions where each term can be differentiated independently.

Composite Function Differentiation

Differentiating composite functions requires a skillful application of the chain rule. A composite function is essentially a function inside another function, and when differentiating, you work from the outside in. It requires patience and precision to ensure each layer is addressed appropriately.

As outlined in the exercise, you first differentiate the outer function, which involves the logarithm, and then you tackle the derivative of the inner quadratic function. The process showcases how the chain rule is effectively utilized to unpack and solve derivatives of functions that are embedded within each other. Mastering this concept is fundamental for in-depth calculus problem-solving.