Chapter 5: Problem 5
Let \(f(x)=(x-3)^{-2}\). Show that there is no value \(c \in(1,4)\) such that \(f^{\prime}(c)=(f(4)-\) \(f(1)) /(4-1) .\) Why is this not a contradiction of the Mean Value Theorem?
Short Answer
Expert verified
There is no such \(c\) because the function is not continuous on \([1,4]\), breaching Mean Value Theorem conditions.
Step by step solution
01
Calculate the Function Values
We need to calculate the value of the function at the given points:\[f(4) = (4-3)^{-2} = 1^{-2} = 1\]\[f(1) = (1-3)^{-2} = (-2)^{-2} = \frac{1}{4}\]
02
Calculate the Average Rate of Change
Now, we'll calculate the average rate of change between \(x = 4\) and \(x = 1\):\[\frac{f(4) - f(1)}{4-1} = \frac{1 - \frac{1}{4}}{3} = \frac{\frac{3}{4}}{3} = \frac{1}{4}\]
03
Find the Derivative of the Function
The next task is to find the derivative of the function \(f(x)\). The function is \((x-3)^{-2}\), so using the power rule for derivatives, we have:\[f'(x) = -2(x-3)^{-3}(1) = \frac{-2}{(x-3)^3}\]
04
Evaluate f'(c) for Values in (1,4)
We want \(f'(c) = \frac{1}{4}\). Thus, we solve:\[\frac{-2}{(c-3)^3} = \frac{1}{4} \Rightarrow -2 = \frac{1}{4}(c-3)^3 \Rightarrow (c-3)^3 = -8 \Rightarrow c-3 = -2 \Rightarrow c = 1\]But \(c = 1\) is not in the interval \((1,4)\). Therefore, there is no value \(c\) in \((1,4)\) such that \( f'(c) = \frac{1}{4} \).
05
Check Mean Value Theorem Conditions
The Mean Value Theorem states that if a function \(f(x)\) is continuous on \([a, b]\) and differentiable on \((a, b)\), then there exists at least one \(c \in (a, b)\) such that \[ f'(c) = \frac{f(b) - f(a)}{b-a} \].However, \(f(x) = (x-3)^{-2}\) is not continuous on \([1, 4]\) because it has a vertical asymptote at \(x = 3\). Thus, it doesn't satisfy the conditions required by the Mean Value Theorem.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Average Rate of Change
An average rate of change measures how a quantity changes on average over a certain interval. It's like calculating the slope between two points on a graph. For a function, consider two points, \(a\) and \(b\), and their corresponding function values \(f(a)\) and \(f(b)\). The formula for the average rate of change is given by:
- \[ \frac{f(b) - f(a)}{b - a} \]
Derivative of a Function
The derivative of a function tells us the rate at which the function's output value changes as its input value changes. In essence, it's a tool for finding instantaneous rates of change—the slope of the tangent line to the curve at any given point. To find the derivative of a function like \(f(x) = (x-3)^{-2}\), we use the power rule:
- The power rule states that if \(f(x) = x^n\), then the derivative \(f'(x) = nx^{n-1}\).
- \[ f'(x) = \frac{-2}{(x-3)^3} \]
Continuity and Differentiability
Continuity and differentiability are crucial conditions for applying the Mean Value Theorem. A function is continuous over an interval if there are no breaks, jumps, or holes in the graph within that interval. Differentiability, on the other hand, means that the function has a defined derivative everywhere in the interval.In the context of the Mean Value Theorem, which requires both continuity on \[a, b\] and differentiability on \( (a, b)\), these two conditions ensure that the function behaves predictably without any abrupt changes.The exercise's function \(f(x) = (x-3)^{-2}\) is not continuous on the specified interval \[1, 4\] due to a vertical asymptote at \(x = 3\). This makes the function neither continuous nor differentiable over \[1, 4\], invalidating the use of the Mean Value Theorem in this context. Without fulfilling these conditions, we cannot guarantee the existence of a point where the instantaneous rate of change equals the average rate of change.
Vertical Asymptote
A vertical asymptote in a function occurs at a particular point where the function heads towards infinity as the input approaches a specific value. In simple terms, it is a vertical line that the graph of the function approaches but never actually touches.For \(f(x) = (x-3)^{-2}\), there is a vertical asymptote at \(x = 3\). As \(x\) inches closer to 3 from either the left or the right, the values of \(f(x)\) shoot up towards infinity. This behavior disrupts continuity, as the function isn’t defined at \(x = 3\). Vertical asymptotes are significant because:
- They indicate points of discontinuity in a function.
- They prevent the application of certain mathematical theorems, like the Mean Value Theorem, across an interval containing the asymptote.