Chapter 5: Problem 20
Compute the average value \(f_{\text {avg }}\) of \(f\) over \([a, b]\) and find a point \(c\) in \((a, b)\) for which \(f(c)=f_{\text {avg. }}\) Illustrate the geometric significance of \(c\) with a sketch accompanied by a description. \(f(x)=2 x+2 / x \quad a=1, b=e\)
Short Answer
Expert verified
The average value is \( \frac{e^2 + 1}{e-1} \) and the point \( c \) satisfies \( 2c + \frac{2}{c} = \frac{e^2 + 1}{e-1} \). The point \( c \) represents where a rectangle matches the area under \( f(x) \).
Step by step solution
01
Write the Formula for the Average Value
The average value of a continuous function \( f \) over the interval \([a, b]\) is given by the formula: \[ f_{\text{avg}} = \frac{1}{b-a} \int_{a}^{b} f(x) \, dx \] Apply this to our function \( f(x) = 2x + \frac{2}{x} \) with \( a = 1 \) and \( b = e. \)
02
Set Up the Integral
Since \( f(x) = 2x + \frac{2}{x} \), set up the integral: \[ \int_{1}^{e} \left( 2x + \frac{2}{x} \right) \, dx \]
03
Integrate the Function
Now, integrate each term separately: \[ \int 2x \, dx = x^2 \quad \text{and} \quad \int \frac{2}{x} \, dx = 2\ln|x| \] Thus, \[ \int_{1}^{e} (2x + \frac{2}{x}) \, dx = \left[ x^2 + 2\ln|x| \right]_{1}^{e} \]
04
Evaluate the Definite Integral
Calculate the evaluation of the integral: \[ \left[ e^2 + 2\ln|e| \right] - \left[ 1^2 + 2\ln|1| \right] = e^2 + 2 - 1 \] Therefore, the result is \( e^2 + 1 \).
05
Calculate the Average Value
Substitute back into the formula for \( f_{\text{avg}} \): \[ f_{\text{avg}} = \frac{1}{e-1} (e^2 + 1) \] This represents the average value of \( f \) over \([1, e]\).
06
Find the Point \( c \) Where \( f(c) = f_{\text{avg}} \)
Set \( f(c) = 2c + \frac{2}{c} = \frac{1}{e-1} (e^2 + 1) \).This is a transcendental equation that may require numerical methods or a calculator for an exact solution.
07
Interpret the Geometric Significance
The point \( c \) for which \( f(c) = f_{\text{avg}} \) geometrically means that the rectangle with area \((b-a)\cdot f_{\text{avg}}\) under the curve \( f(x) \) fits exactly under the curve \( f(x) \) over the interval \([a, b]\). Sketching \( f(x) \) from 1 to \( e \), the rectangle at height \( f_{\text{avg}} \) covers the same total area as the area under \( f(x) \).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Definite Integral
A definite integral is a crucial concept in calculus, particularly when dealing with functions over a specific interval. It essentially represents the total accumulated value, or the net area, under a curve described by a function over a given interval \([a, b]\).This involves the process of integrating the function from point \(a\) to point \(b\) on the x-axis. The integral of a function \(f(x)\) can be thought of as summing up infinitely many infinitesimally small areas under the curve.
- The notation \(\int_{a}^{b} f(x) \, dx\) denotes the integral of \(f(x)\) from \(a\) to \(b\).
- The result of this integration is a single number that represents the total area under the curve and above the x-axis between \(x = a\) and \(x = b\).
Geometric Interpretation
Understanding the geometric interpretation of the average value of a function enriches your grasp of calculus. When we talk about average value, it's not just a number; it's a way to conceptualize how a function behaves over an interval.Picture plotting the function \(f(x) = 2x + \frac{2}{x}\) on a graph from 1 to \(e\). The average value of this function represents the height of a rectangle that would, over the same interval, cover the same amount of area as the area under \(f(x)\). Imagine a flat, straight-edged rectangle compared to the flowing curve. The point \(c\) where \(f(c) = f_{\text{avg}}\) is significant because it shows where the function's value equals its average. At this point, the curve touches the rectangle's height, effectively showcasing where the function balances out over its domain.
- Geometrically, it helps illustrate how areas under curves relate to more straightforward forms, like rectangles.
- It also offers insight into the balance point of the function over the interval, providing a different perspective on integration and function analysis.
Transcendental Equation
A transcendental equation is an equation involving transcendental functions, which are non-algebraic and cannot be expressed as a finite sequence of algebraic operations (such as polynomials, n-th roots, etc.). These include functions like exponential, logarithmic, trigonometric functions, among others.In the exercise at hand, the equation \(2c + \frac{2}{c} = \frac{1}{e-1} (e^2 + 1)\) is transcendental due to its complexity and the presence of division involving variables. Solving transcendental equations often requires numerical methods or tools like a graphing calculator as there's no straightforward "solve-for-x" process as with algebraic equations.
- These equations are essential in many fields, such as physics and engineering, where they model more complex phenomena that algebraic equations can't accurately represent.
- In calculus, transcendental equations present challenges that test one's problem-solving skills and understanding of numerical approximation techniques.