3.6 - Worked Example
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Using Central Difference Formula
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Today, we’re going to explore how to use the central difference formula to estimate a derivative. Can anyone remind me what the central difference formula is?
Isn't it something like f'(x) = (f(x+h) - f(x-h)) / (2h)?
Exactly! We use this formula when we have function values at discrete points. Now, what does h represent?
It's the difference between the x values.
Yes! h is essentially the step size between the points. In our worked example, we’ll be using the values provided to estimate the derivative at x = 1.4. Let's look at the data table provided.
So we need to find f(1.6) and f(1.2) for this, right?
Correct! Now, who can tell the class the values of f(1.6) and f(1.2) based on the table?
f(1.6) is 1.296 and f(1.2) is 0.128.
Great job! Now, let’s plug those into the formula. What do we calculate next?
We calculate f'(1.4) = (1.296 - 0.128) / (2 * 0.2).
Correct! So, what does that give us?
That’s 1.168 divided by 0.4, which equals 2.92.
Wonderful! So, our estimated derivative f′(1.4) is approximately 2.92. Who can summarize why we used the central difference method here?
We used it because we had discrete data points and wanted to estimate the derivative.
Exactly! Great work today, everyone!
Understanding Errors in Numerical Differentiation
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Now that we’ve completed our worked example, let’s discuss potential errors in numerical differentiation. Can anyone think of a source of error?
There could be round-off errors due to precision limits?
Correct! Round-off errors can impact our calculations, especially if h is very small. What about truncation error?
That happens when we ignore higher-order terms in our formulas?
Exactly! It’s vital to consider these errors when choosing your method. For our example, how might the choice of h impact our results?
If h is too small, we might encounter bigger round-off errors, affecting accuracy?
Right again! Careful selection of h leads to more reliable outputs. Let’s keep these points in mind when applying these techniques in future exercises.
So it’s important to balance how small we choose h and how accurate we want our result?
Spot on! Keep this in mind to enhance the accuracy of your numerical differentiation.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
The section provides a specific worked example using the central difference formula to compute the first derivative of a function at a certain point. This helps demonstrate the application of numerical differentiation techniques when only discrete data points are available.
Detailed
Worked Example in Numerical Differentiation
In this section, we delve into a practical worked example that illustrates the application of numerical differentiation methods. Given a table of values for a function, we will compute the derivative using the central difference formula. The scenario presented assumes that the function values are known at discrete points and we are particularly focused on estimating the derivative at one of these points—specifically, at 𝑥 = 1.4. The method used here emphasizes the procedure and results, shedding light on the process of applying central difference in numerical differentiation.
Given the table:
| x | f(x) |
|---|---|
| 1 | 0 |
| 1.2 | 0.128 |
| 1.4 | 0.544 |
| 1.6 | 1.296 |
| 1.8 | 2.432 |
We start by determining the step size ℎ as the difference between the points:
ℎ = 1.4 - 1.2 = 0.2
Using the central difference formula:
$$f'(x) ≈ \frac{f(x + h) - f(x - h)}{2h}$$
We substitute the values as follows:
- $$f'(1.4) ≈ \frac{f(1.6) - f(1.2)}{2(0.2)}$$
- $$= \frac{1.296 - 0.128}{0.4} ≈ 2.92$$
Thus, we find that the estimated value of the derivative at 𝑥 = 1.4 is approximately 2.92, providing a clear application of the central difference method in numerical differentiation.
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Given Table of Values
Chapter 1 of 4
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Chapter Content
Given Table:
| x | f(x) |
|---|---|
| 1 | 0 |
| 1.2 | 0.128 |
| 1.4 | 0.544 |
| 1.6 | 1.296 |
| 1.8 | 2.432 |
Detailed Explanation
This table presents discrete data points where the function f(x) has been evaluated at specific x values. Each x value corresponds to a specific f(x) value, which is the output of the function at that input point. For example, when x is 1.2, the function value is 0.128.
Examples & Analogies
Think of the table as a list of measurements taken during an experiment. Just like scientists record their results at certain intervals, this table shows how the function behaves at specific x values.
Selecting the Step Size (h)
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Here, ℎ = 0.2
Detailed Explanation
In this example, the value of h is defined as 0.2. This value represents the uniform distance between the x values in the provided table. It can be calculated by subtracting consecutive x values, for example, 1.2 - 1.0 = 0.2.
Examples & Analogies
Imagine you're measuring the height of plants every two days. If you measure on day 1, the next measurement would be on day 3. The interval (step size) between your measurements is like h.
Calculating the Central Difference
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𝑓′(1.4) ≈ \( \frac{𝑓(1.6)−𝑓(1.2)}{2ℎ} = \frac{1.296− 0.128}{0.4} = 2.92 \)
Detailed Explanation
To find the derivative of the function at x = 1.4, we apply the central difference formula. We take the value of the function at 1.6 (which is 1.296) and subtract the value at 1.2 (which is 0.128). This gives us 1.168. We then divide by 2 multiplied by h (0.4) to find the approximate rate of change (derivative) of the function at that point.
Examples & Analogies
Imagine you are trying to determine how fast a car is moving at a specific moment. If you know the car's distance at two close points in time, you can find the speed (which is a derivative) by calculating the change in distance over the change in time, similar to how we calculated the derivative here.
Final Result
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So, 𝑓′(1.4)≈ 2.92
Detailed Explanation
Once we perform the calculation, we find that the approximate derivative of the function at x = 1.4 is 2.92. This means that at this specific point, the function is rising at a rate of approximately 2.92 units of f(x) for every 1 unit increase in x.
Examples & Analogies
Continuing with the car analogy, if the result is 2.92, it means when the car is at a particular position (x = 1.4), it is accelerating or moving upward fairly quickly — a speed of 2.92 is relatively fast for a car.
Key Concepts
-
Numerical Differentiation: A technique used for estimating function derivatives from discrete data.
-
Central Difference Method: A specific finite difference method for calculating derivatives.
-
Step Size (h): The gap between consecutive data points, crucial for accurate differentiation.
-
Truncation Error: Error from omitting higher-order derivative terms.
-
Round-off Error: Errors arising from numerical precision limits during calculations.
Examples & Applications
Using a table of values, the central difference formula can be applied to find f'(1.4) by using data points from the table.
When estimating the derivative at x = 2, different approaches can yield varying accuracy based on the selection of h.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
When f at points you want to find, use h to give you peace of mind.
Stories
Imagine a race where runners start at different points; you need to guess their speed based on how far apart they are.
Memory Tools
D.E.A.R: Differentiate using Estimates And Ratios (helps to remember the essence of numerical differentiation).
Acronyms
C.D.M
Central Differencing Method (reminds students of what the central difference technique is).
Flash Cards
Glossary
- Numerical Differentiation
A method for estimating the derivatives of a function using discrete data points.
- Central Difference
A finite difference method used to estimate the derivative of a function at a certain point.
- Step Size (h)
The uniform distance between two consecutive data points in the discrete data set.
- Truncation Error
Error caused by ignoring higher-order terms in numerical formulas.
- Roundoff Error
Error caused by the limited precision of numerical calculations.
Reference links
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