Chapter 4: Problem 46
Use the guidelines of this section to make a complete graph of \(f\). $$f(x)=e^{-x^{2} / 2}$$
Short Answer
Expert verified
The key features of this function include the following:
1. Domain: \((-\infty, +\infty)\)
2. Range: \((0, 1]\)
3. Increasing interval: \((-\infty, 0)\)
4. Decreasing interval: \((0, +\infty)\)
5. Local maximum at (0, 1)
6. Horizontal asymptote: y=0
7. Concave up: \((-\infty, -1)\cup(1, \infty)\)
8. Concave down: \((-1, 1)\)
Step by step solution
01
Determine the Domain
The domain of a function is the set of all possible input values (x-values) for which the function is defined. Since the exponent in the function is a real number (namely, \(-x^2/2\)), there are no restrictions on the domain. Therefore, the domain of the function is:
Domain: \((-\infty, +\infty)\)
02
Determine the Range
The range of a function is the set of all possible output values (y-values) produced by the function. Since the base of the exponential function is the number "e" (which is always positive) and the exponent will never be positive (\(-x^2/2\) is always non-positive), the output values of the function will always be between 0 and 1 (excluding 0). Thus, the range of the function is:
Range: \((0, 1]\)
03
Identify Increasing and Decreasing Intervals
To determine the increasing and decreasing intervals, we need to first find the derivative of the function.
$$f'(x)=\frac{d}{dx}(e^{-x^2/2})$$
Using the chain rule, we get:
$$f'(x)=e^{-x^2/2}\cdot(-x)= -xe^{-x^2/2}$$
Now, we need to find the critical points where the derivative is equal to 0 or undefined. In this case, the derivative is never undefined (since it is an exponential function). So, we only need to find where \(f'(x)=0\). Since \(e^{-x^2/2}\) is always positive, we can focus on the other part: \(-x=0\). The only critical point is at \(x=0\).
Now, we can determine the intervals where the function is increasing and decreasing:
Increasing interval: \((-\infty, 0)\)
Decreasing interval: \((0, +\infty)\)
04
Identify the Local Maximum
Since there is only one critical point (\(x=0\)), and we know the function is increasing before the critical point and decreasing after, we can conclude that there is a local maximum at \(x=0\). To find the y-value of this local maximum, we can plug \(x=0\) into the original function \(f(x)\):
\(f(0)=e^{-0^2/2}=e^0=1\)
Thus, there is a local maximum at the point \((0,1)\).
05
Identify Asymptotes and Concavity
Since the function is an exponential function, there is a horizontal asymptote at y=0. There are no vertical asymptotes.
To determine the concavity, we need to find the second derivative:
$$f''(x)=\frac{d^2}{dx^2}\left(-xe^{-x^2/2}\right)$$
Applying the product rule, we get:
$$f''(x)=-e^{-x^2/2}+x^2 e^{-x^2/2}$$
We identify the inflection points (where concavity changes) by finding where the second derivative is equal to 0 or undefined. Again, we only need to look at where it is equal to 0. Notice that the exponential term is always positive, so we only need to look at the other terms:
$$0=-1+x^2\implies x=\pm\sqrt{1}\implies x=\pm1$$
We have two inflection points at \(x=-1\) and \(x=1\). Using these points, we can determine the concavity intervals:
Concave up: \((-\infty, -1)\cup(1, \infty)\)
Concave down: \((-1, 1)\)
06
Graphing the Function
Using all the information we gathered in the previous steps, the graph of the function \(f(x)=e^{-x^2/2}\) will have the following features:
1. Domain: \((-\infty, +\infty)\)
2. Range: \((0, 1]\)
3. Increasing interval: \((-\infty, 0)\)
4. Decreasing interval: \((0, +\infty)\)
5. Local maximum at (0, 1)
6. Horizontal asymptote: y=0
7. Concave up: \((-\infty, -1)\cup(1, \infty)\)
8. Concave down: \((-1, 1)\)
Now, you can draw the complete graph of the function \(f(x)=e^{-x^2/2}\) using the gathered information.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Function Domain
Understanding the domain of a function is essential because it tells us the possible x-values we can use in that function. Consider the function \( f(x) = e^{-x^2/2} \). Here, the input, or x-value, is squared and multiplied by a constant before being used as an exponent for the base \( e \).
Since \( e \) raised to any real number is always defined, there are no restrictions on input values. This implies the domain of the function is all real numbers:
Since \( e \) raised to any real number is always defined, there are no restrictions on input values. This implies the domain of the function is all real numbers:
- Domain: \(( -\infty, +\infty )\)
Increasing and Decreasing Intervals
Determining where a function is increasing or decreasing helps understand its behavior over its domain. With \( f(x) = e^{-x^2/2} \), we use the first derivative, \( f'(x) = -x e^{-x^2/2} \), to find these intervals. By setting \( f'(x) \) to zero, we find that the critical point occurs at \( x = 0 \).
To determine the nature of the intervals, observe the sign of the derivative:
- For \( x < 0 \), \( f'(x) > 0 \) indicating the function is increasing on \( (-\infty, 0) \).
- For \( x > 0 \), \( f'(x) < 0 \) indicating the function is decreasing on \( (0, +\infty) \).
This tells us how the function behaves around the critical point and how the function climbs to a peak and then falls.
To determine the nature of the intervals, observe the sign of the derivative:
- For \( x < 0 \), \( f'(x) > 0 \) indicating the function is increasing on \( (-\infty, 0) \).
- For \( x > 0 \), \( f'(x) < 0 \) indicating the function is decreasing on \( (0, +\infty) \).
This tells us how the function behaves around the critical point and how the function climbs to a peak and then falls.
Local Maximum and Minimum
A local maximum or minimum occurs at a point where the function changes from increasing to decreasing or vice versa. Given \( f(x) = e^{-x^2/2} \), the analysis of the first derivative shows that the function has a local maximum where it shifts from increasing to decreasing.
At \( x = 0 \), the function \( f(x) \) hits this local maximum. By substituting \( x = 0 \) back into the function, we find \( f(0) = 1 \). Therefore, the local maximum point is at:
At \( x = 0 \), the function \( f(x) \) hits this local maximum. By substituting \( x = 0 \) back into the function, we find \( f(0) = 1 \). Therefore, the local maximum point is at:
- Local Maximum: (0, 1)
Concavity and Inflection Points
Concavity tells us whether a function is curving upwards or downwards at any given point, which can help to visualize the graph more accurately. The second derivative \( f''(x) = -e^{-x^2/2} + x^2 e^{-x^2/2} \) reveals areas of concavity.
To find inflection points, we set \( f''(x) = 0 \):
- Solving \( -1 + x^2 = 0 \) results in \( x = \pm 1 \).
At these points, the function changes concavity, meaning the graph changes its curve direction.
The intervals can be summarized as follows:
To find inflection points, we set \( f''(x) = 0 \):
- Solving \( -1 + x^2 = 0 \) results in \( x = \pm 1 \).
At these points, the function changes concavity, meaning the graph changes its curve direction.
The intervals can be summarized as follows:
- Concave up: \(( -\infty, -1 ) \cup ( 1, \infty )\)
- Concave down: \(( -1, 1 )\)