How to Find Average Rate of Change of a Function – Your Step-by-Step Guide

How to Find Average Rate of Change of a Function Your Step-by-Step Guide

To find the average rate of change of a function, you should first identify two distinct points on the function and note their coordinates.

The average rate of change is essentially the slope of the secant line that intersects the graph of the function at these points. In calculus, this concept helps us understand how a function‘s output value changes in response to changes in the input value over a certain interval.

You calculate it using the formula $\frac{f(b) – f(a)}{b – a}$, where $a$ and $b$ are the input values at the two points and $f(a)$, $f(b)$ are the corresponding output values from the function.

This computation will give you the rate of change per unit, on average, over the interval from $a$ to $b$.

Remember, digging into this idea opens the door to predicting how things change over time or space in various scientific and mathematical contexts. Stay with me, and let’s explore the intriguing world of change together.

Calculating Average Rate of Change of a Function

When I want to measure how a function’s output changes relative to its input, I calculate its average rate of change over a specific interval. This is akin to finding the average speed of a car over a road trip.

Step-by-step Procedure

  1. Identify the Interval: Select the range of x values (the input) over which you want to determine the average rate of change. These are often referred to as the endpoints, ( a ) and ( b ) respectively.

  2. Calculate Change in Output $\Delta y $: Find the function values at these endpoints, ( f(a) ) and ( f(b) ). The change in output, $\Delta y$, is ( f(b) – f(a) ).

  3. Calculate Change in Input $ \Delta x $: The change in input, $\Delta x$, is ( b – a ).

  4. Use the Slope Formula: The average rate of change is analogous to the slope of the secant line that connects the endpoints (( a, f(a) )) and (( b, f(b) )) on the function’s graph. Calculate the slope with the formula $\frac{\Delta y}{\Delta x}$.

  5. Evaluate the Result: Insert the values into the slope formula to get $\frac{f(b) – f(a)}{b – a}$. The resulting value is your function’s average rate of change.

For a tangible example, if I’m looking at a population change over time, the average rate of change tells me how much the population grew or diminished per year on average over a specific time frame.

StepOperationExample Calculation
1Select ( a ) and ( b )( a = 2000 ), ( b = 2010 )
2Find ( f(a) ) and ( f(b) )Population ( f(a) = 50,000 ), ( f(b) = 70,000 )
3Calculate $ \Delta y$$ \Delta y = 70,000 – 50,000 = 20,000 $ people
4Calculate $\Delta x$$ \Delta x = 2010 – 2000 = 10 $ years
5Apply formulaAverage rate $ = \frac{20,000}{10} = 2,000 $ people/year

Remember, the sign of the average rate of change implies whether the function is increasing, decreasing, or remaining constant over the interval.

A positive value signifies an increasing trend, a negative one indicates a decreasing trend and a zero means the output is constant, no matter the change in input.


In mastering the concept of average rate of change, I have learned to view functions dynamically. The average rate of change is akin to measuring the slope between two points on a graph.

Specifically, it quantifies how the output of a function changes concerning changes in the input over an interval. To calculate, I use the formula:

$$ \text{Average rate of change} = \frac{f(x_2) – f(x_1)}{x_2 – x_1} $$

Remember, $x_1 $ and $ x_2 $ are the input values, while $f(x_1) $ and $f(x_2) $ are the respective outputs from the function ( f(x) ). This formula gives me a precise rate at which the function moves from one point to another.

By applying this knowledge, I can predict future behavior of a function within a certain interval, assuming the rate remains consistent. This is particularly useful in fields like physics for velocity, or economics for growth rates.

I make sure to interpret the result with context; a positive rate indicates an increasing function, while a negative rate suggests a decrease over the selected interval.

Understanding the average rate of change provides a solid foundation for further exploration in calculus, such as approaching the concept of instantaneous rate of change and eventually the derivative, which give me insights into how a function behaves at any given point.