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Ch. 4 - Applications of Derivatives
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Non è quello che usi tu?Cambia libro di testo
Capitolo 4, Problema 4.1.52b

Theory and Examples


In Exercises 51 and 52, give reasons for your answers.


Let f(x) = |x³ − 9x|.


b. Does f'(3) exist?

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To determine if f'(3) exists, we need to check if the function f(x) = |x³ − 9x| is differentiable at x = 3. Differentiability requires the function to be continuous and have a defined derivative at that point.
First, check the continuity of f(x) at x = 3. Since f(x) is an absolute value function, it is continuous everywhere, including at x = 3.
Next, consider the definition of the derivative: f'(x) = lim (h -> 0) [(f(x + h) - f(x)) / h]. We need to evaluate this limit at x = 3.
To evaluate the derivative, consider the piecewise nature of the absolute value function. For x³ - 9x, identify the intervals where the expression inside the absolute value is positive or negative. This will help in determining the left-hand and right-hand derivatives at x = 3.
Calculate the left-hand derivative (approaching from the left of x = 3) and the right-hand derivative (approaching from the right of x = 3). If both derivatives exist and are equal, then f'(3) exists. If they are not equal, f'(3) does not exist.

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Derivative

The derivative of a function at a point measures the rate at which the function's value changes as its input changes. It is the slope of the tangent line to the function's graph at that point. For a function f(x), the derivative f'(x) is found using limits, and it exists at a point if the function is continuous and smooth (differentiable) there.
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Absolute Value Function

The absolute value function, denoted as |x|, outputs the non-negative value of x, regardless of its sign. When dealing with derivatives, the presence of an absolute value can create points where the function is not differentiable, typically at points where the expression inside the absolute value changes sign, leading to a cusp or corner in the graph.
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Percorso guidato
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Average Value of a Function

Differentiability and Continuity

A function is differentiable at a point if it is smooth and has no sharp corners or cusps at that point, which also implies it must be continuous there. For f(x) = |x³ − 9x|, checking differentiability at x = 3 involves ensuring the function is continuous and that the left-hand and right-hand derivatives at x = 3 are equal.
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Intro to Continuity
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Identifying Extrema


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Theory and Examples


Sketch the graph of a differentiable function y = f(x) that has a local maximum at (1, 1) and a local minimum at (3, 3).

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Analyzing Functions from Derivatives


Answer the following questions about the functions whose derivatives are given in Exercises 1–14:



b. On what open intervals is f increasing or decreasing?


f′(x) = (x − 1)(x + 2)(x − 3)

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