Problem 15 - Calculus and its applications

Question

a) Graph the function. b) Draw tangent lines to the graph at points whose \(x\) -coordinates are \(-2,0,\) and 1 c) Find \(f^{\prime}(x)\) by determining \(\lim _{h \rightarrow 0} \frac{f(x+h)-f(x)}{h}\). d) Find \(f^{\prime}(-2), f^{\prime}(0),\) and \(f^{\prime}(1) .\) These slopes should match those of the lines you drew in part (b). $$f(x)=\frac{1}{x}$$

Step-by-Step Solution

Answer

The derivative function is \(-\frac{1}{x^2}\), with slopes \(-\frac{1}{4}\), undefined, and \(-1\) at \(x = -2, 0, 1\), respectively.

Step 1: Graph the Function

First, we need to graph the function \( f(x) = \frac{1}{x} \). This is a hyperbola with a vertical asymptote at \( x = 0 \) and a horizontal asymptote at \( y = 0 \). The graph will be in two parts: one in the first quadrant and another in the third quadrant, decreasing as \( x \) moves away from zero.

Step 2: Draw Tangent Lines

Next, we draw the tangent lines to the graph at the given \( x \)-coordinates: \(-2\), \(0\), and \(1\). Recall that the tangent line to a function at a given point has the slope equal to the derivative of the function at that point. We'll calculate these slopes in a later step. For now, try to visually estimate and draw lines that just touch the curve at \( x = -2, 0, \) and \( 1 \) without crossing it.

Step 3: Define and Simplify the Derivative

Now, we find \( f'(x) \) by using the definition of the derivative: \[ f'(x) = \lim_{h \to 0} \frac{f(x+h) - f(x)}{h}. \]Substituting \( f(x) = \frac{1}{x} \): \[ f'(x) = \lim_{h \to 0} \frac{\frac{1}{x+h} - \frac{1}{x}}{h}. \]Simplify the expression:\[ f'(x) = \lim_{h \to 0} \left( \frac{x - (x+h)}{h(x+h)x} \right) = \lim_{h \to 0} \left( \frac{-h}{hx(x+h)} \right). \]Cancel \( h \) from the numerator and denominator:\[ f'(x) = \lim_{h \to 0} \left( -\frac{1}{x(x+h)} \right) = -\frac{1}{x^2}. \]

Step 4: Calculate Specific Derivative Values

Finally, we calculate the derivative at the specified points:- \( f'(-2) = -\frac{1}{(-2)^2} = -\frac{1}{4} \).- \( f'(0) \) is undefined because \( -\frac{1}{0^2} \) leads to division by zero, which indicates a vertical tangent or vertical asymptote.- \( f'(1) = -\frac{1}{1^2} = -1 \).These slopes match the tangent lines drawn in Step 2.

Key Concepts

Derivatives

Derivatives play a crucial role in calculus, helping us understand how functions change at any given point. The derivative of a function measures the rate at which the function's value changes as its input changes. Essentially, it tells us the slope of the tangent line to the function at any specific point. If we denote a function by \( f(x) \), its derivative is denoted by \( f'(x) \). This is calculated using the limit definition of the derivative:

• We consider the difference quotient, \( \frac{f(x+h) - f(x)}{h} \), which represents the average rate of change over the interval from \(x\) to \(x+h\).
• To find the instantaneous rate of change at \(x\), we take the limit as \(h\) approaches zero, resulting in the formal derivative: \( f'(x) = \lim_{h \to 0} \frac{f(x+h) - f(x)}{h} \).

Understanding derivatives is pivotal for analyzing function behavior, optimizing solutions, and solving real-world problems in physics, engineering, and more. They help predict trends and make informed decisions in various scientific fields.

Tangent Lines

A tangent line to a function at a given point is a straight line that just "touches" the curve at that point. This line gives the best linear approximation of the function near that point. A tangent line has a unique feature: its slope is equal to the derivative of the function at that point. Here’s a closer look at tangent lines:

• To draw a tangent line to a curve at a specific point \((x, y)\), you can use the point-slope form \( y - y_1 = m(x - x_1) \), where \( m \) is the derivative \( f'(x) \).
• The tangent line determines the behavior of a function near the point of tangency, providing insights into whether the function is increasing or decreasing.
• In our exercise, for example, at \(x = 1\), the slope of the tangent line \( f'(1) \) is \(-1\), forming a line that touches the curve and points downwards.

Identifying and drawing tangent lines is essential for understanding the graphical representation of functions and their local properties.

Limits

The concept of a limit is foundational in calculus, building the groundwork for derivatives and integrals. A limit examines the behavior of a function as the input approaches a certain value. This is crucial for determining the derivative.

• A limit \( \lim_{x \to a} f(x) = L \) indicates that as \( x \) gets closer to \( a \), \( f(x) \) approaches \( L \).
• For derivatives, the focus is on \( h \rightarrow 0 \) in the difference quotient, allowing us to evaluate how \( f(x) \) changes instantaneously, rather than over a finite interval.

In the exercise given, calculating the derivative of \( f(x) = \frac{1}{x} \) involves using limits to simplify from a complex fraction to \( f'(x) = -\frac{1}{x^2} \). Limits help in overcoming indeterminate forms and provide a precise way to describe function behavior at points of interest.

Graphing Functions

Graphing functions is a vital skill in calculus, providing a visual comprehension of function behavior. It allows us to see trends, identify key features, and understand complex relationships. Here's a breakdown:

• The graph of a function \( f(x) = \frac{1}{x} \) is a classic example of a hyperbola, with two branches representing different value ranges.
• This function features a vertical asymptote at \( x = 0 \) (as the function is undefined at that point) and a horizontal asymptote at \( y = 0 \), demonstrating how the function decreases towards zero as \( x \) approaches infinity.
• Tangent lines drawn on this graph help illustrate the derivative's meaning as the slope at specific points, such as \( -2 \), \( 0 \), and \( 1 \).

Graphing enhances understanding by correlating algebraic manipulation with visual insights, helping us predict and interpret how functions behave over different intervals.