Problem 25 - Calculus - AP Edition

Question

Use a graphing utility to estimate the value of \(f^{\prime}(1)\) by zooming in on the graph of \(f\), and then compare your estimate to the exact value obtained by differentiating. $$f(x)=\frac{x}{x^{2}+1}$$

Step-by-Step Solution

Answer

f'(1) = 0

Step 1: Differentiate using the quotient rule

\(f'(x) = \frac{(x^2+1) - x(2x)}{(x^2+1)^2} = \frac{1-x^2}{(x^2+1)^2}\)

Step 2: Evaluate at x = 1

\(f'(1) = \frac{1-1}{(1+1)^2} = \frac{0}{4} = 0\)

This can also be verified graphically: \(f\) has a local maximum at \(x = 1\), where the tangent line is horizontal.

Key Concepts

Graphing Utilities

Graphing utilities are powerful tools for visualizing mathematical functions. They help students and professionals alike to understand complex equations by offering a visual representation. These tools can plot functions, show trends, and provide insights about the behavior of functions at specific points.
If you consider the function given by the exercise, which is \(f(x) = \frac{x}{x^2 + 1}\), using a graphing utility like Desmos or a graphing calculator can tremendously aid in understanding its characteristics.

• First, input the function into the utility.
• Then, observe how the graph behaves, especially around the point \(x = 1\).
• This step helps visualize where the function increases or decreases and how sharply.

Graphing utilities not only simplify the process of plotting functions but also provide detailed visuals when zooming in and out. This is particularly useful when you need to estimate derivatives by closely examining the curve.

Estimating Derivatives

Estimating derivatives involves approximating the slope of a tangent line to a curve at a particular point. By using graphing utilities, you can focus on the region around that specific point.
The process begins by gradually zooming in on the point of interest, which, in this case, is \(x = 1\) on the graph of \(f(x) = \frac{x}{x^2 + 1}\). As we get closer and closer to the curve, it tends to appear more linear.
Here's how you can estimate the derivative:

• Identify two very close points on the function near \(x = 1\).
• Calculate the difference in \(y\)-values (output of the function) between these points.
• Divide this difference by the difference in \(x\)-values (input of the function).

This provides an approximation of the derivative, \(f'(x)\), at \(x = 1\). As we zoom further, the estimated slope should get closer to the true derivative as determined by calculus.

Derivative at a Point

The derivative of a function at a particular point signifies the instantaneous rate of change or the slope of the tangent line at that specific point. To find this exactly for \(f(x) = \frac{x}{x^2 + 1}\) at \(x = 1\), we use differentiation.
The exact derivative \(f'(x)\) can be computed by applying differentiation rules, such as the quotient rule. For the function parting into quotient form, use:

• The quotient rule: If \(h(x) = \frac{u(x)}{v(x)}\), then \(h'(x) = \frac{u'(x)v(x) - u(x)v'(x)}{v(x)^2}\).

For \(f(x) = \frac{x}{x^2 + 1}\), substitute:

• \(u(x) = x\) and \(v(x) = x^2 + 1\).
• Differentiating these separately gives \(u'(x) = 1\) and \(v'(x) = 2x\).
• Plug them into the quotient rule formula to obtain \(f'(x)\).

Finally, substitute \(x = 1\) into the derived function \(f'(x)\) to get the exact derivative at \(x = 1\). This exact value can then be compared with the estimated value from graphing utilities to check accuracy.