How to Find the Average Rate of Change From Tables, Graphs, and Functions

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Easy instructions for finding the average rate of change

Co-authored by Carmine Shannon Reviewed by Joseph Meyer

Last Updated: April 20, 2024 Fact Checked

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Average Rate of Change of a Function

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Average Rate of Change From a Graph

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Average Rate of Change From a Table

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Estimating Instantaneous Rate of Change

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When to Use Average Rate of Change

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Video

The average rate of change is the slope of a line that intersects two points on a curve. Finding it is a classic pre-calculus problem, and it’s as simple as calculating the slope of a straight line! All you need to do is find your x and y values for both points, subtract the first y value from the second, and divide it by the second x value minus the first.

Equation for Average Rate of Change

The equation for the average rate of change is ${\frac {y2-y1}{x2-x1}}$ , or the difference in y values divided by the difference in x values.

Section 1 of 5:

Average Rate of Change of a Function

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1
Determine your x and y values. The average rate of change is the slope of a line that intersects two points on a curve. Since it’s just a straight line, it’s the change in y divided by the change in x. ^{[1]
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Research source} If you’re given the interval [x ₁ , x ₂ ], plug each value into your equation to get y ₁ and y ₂ .
- For example, if you had the function f(x) = x ² + 3x and the interval [1, 3], then f(1) = 1 ² + 3 * 1 = 1 + 3 = 4 and f(3) = 3 ² + 3 * 3 = 9 + 9 = 18. So your x values are 1 and 3 and your y values are 4 and 18.
 - The variable y is also referred to as f(x) since it’s the value you get when you plug an x value into the function.
2
Subtract y ₁ from y ₂ and x ₁ from x ₂ . The equation for the average rate of change is ${\frac {y2-y1}{x2-x1}}$ . To get your numerator, subtract y ₁ from y ₂ (AKA f(x ₁ ) from f(x ₂ )). Then find your denominator by subtracting x ₁ from x ₂ . ^{[2]
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- For the function f(x) = x ² + 3x over the interval [1, 3], the fraction would look like ${\frac {18-4}{3-1}}$ = ${\frac {14}{2}}$ .
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3
Divide the change in y by the change in x. Once you’ve set up your fraction, divide the numerator by the denominator. This gives you the average rate of change, which is also the slope of something called a secant line. ^{[3]
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- The average rate of change for the example function is ${\frac {14}{2}}$ = 7.

Section 2 of 5:

Average Rate of Change From a Graph

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1
Use the graph to find your x and y values. To find the x value of a point on a graph, draw a straight line from the point to the x-axis—you will either have to draw straight up or straight down. Where your line intersects the axis is your value. Find y by drawing a straight line to the y-axis, either to the left or right of the point. ^{[4]
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- If your y value is negative, draw straight up for x. If it’s positive, draw straight down. If the x value is negative, draw right for y, and if it’s positive draw left.
2
Determine the values for y ₁ , y ₂ , x ₁ and x ₂ . Choose one set of points to be (x ₁ , y ₁ ) and the other to be (x ₂ , y ₂ ). You can choose either, but usually, people put the leftmost coordinates first (lowest value of x). ^{[5]
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- So if you had the points (1, 3) and (5, 10), 1 would be x ₁ and 3 would be y ₁ .
- If you put the right coordinates first your numerator and denominator will switch from positive to negative, or from negative to positive. However, the equation gives you the same answer either way.
3
Divide the difference of the y’s by the difference of the x’s. Use the equation ${\frac {y2-y1}{x2-x1}}$ to find the average rate of change. ^{[6]
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- So if you had (1, 3) and (5, 10), the slope would be ${\frac {10-3}{5-1}}$ = ${\frac {7}{4}}$ .

Section 3 of 5:

Average Rate of Change From a Table

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1
Find your y or x values, given a certain interval. If you’re asked to find the rate of change of an interval with respect to x (AKA the interval is from one x value to another), find the x values on the table. Next to the x values are their corresponding y values. ^{[7]
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- It’s less common to be given an interval with respect to y (two y values). If you find yourself in that situation, do the same thing, but look for the given values in the y column, instead.
2

Use the equation ${\frac {y2-y1}{x2-x1}}$ to find the average rate of change. Subtract the y value that corresponds with the first x value from the y value that corresponds to the second x value. Then, subtract the first x value from the second, and divide the difference in y by the difference in x. ^{[8]
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Section 4 of 5:

Estimating Instantaneous Rate of Change

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The average rate of change is the same as the slope of a secant line, or a line that goes through two points of a curve. The instant rate of change is a tangent line, which only touches a single point. ^{[9]
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Research source} Choose two points that are close to where you want to find the instantaneous rate of change.
2
Calculate the average rate of change over the two points. Use the equation ${\frac {y2-y1}{x2-x1}}$ to find the slope of the secant line that intersects the two points. ^{[10]
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- If you wanted to find the instantaneous rate of change for f(x) = x ² + 4x when x = 2, then you could make your first secant line at the points (1, 5) and (3, 21) The slope of that line is ${\frac {21-5}{3-1}}$ , or 8.
3
Choose two more points, both closer to the target point. Find the average rate of change over an interval even closer to where you want to find the instantaneous rate of change. ^{[11]
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Research source} If you have exponents in your function you will likely need to use a calculator.
- Try the rate of change at one decimal place on either side of your target. For the equation (x) = x ² + 4x, you could use x ₁ = 1.9 and x ₂ = 2.1. ${\frac {12.81-11.21}{2.1-1.9}}$ = ${\frac {1.6}{.2}}$ = 8.
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Estimate the tangent line based on the secant lines. Keep finding secant lines closer and closer to the point, until you start to see your results converge on a number. ^{[12]
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Research source} In the example above, a good guess would be that the instantaneous rate of change at x = 2 for f(x) = x ² + 4x is 8.