How to Tell if a Function Is Increasing or Decreasing – Identifying Slopes Made Simple

How to Tell if a Function Is Increasing or Decreasing Identifying Slopes Made Simple

To tell if a function is increasing or decreasing, you should first understand the slope of the function over a given interval.

If you’re examining a function ( f(x) ), you can determine if it’s increasing by checking if ( f'(x) > 0 ) for all ( x ) within the interval.

Conversely, the function is decreasing when ( f'(x) < 0 ) over that same range. This approach allows you to pinpoint where the function goes upwards or downwards as the variable ( x ) changes values.

Knowing when a function ascends or descends is a fundamental concept in calculus and helps provide a deeper comprehension of the behavior of mathematical models. With a clear organization of thought and a step-by-step reading of the content, this seemingly complex topic becomes much more approachable.

As this article unfolds, I’ll guide you through the subtleties of analyzing functions so that you can apply this knowledge confidently and with ease.

Stick with me, and you’ll see how simple it can be to apply these rules to various functions and appreciate the beautiful logic that governs their behavior.

Steps for Determining if a Function is Increasing or Decreasing

When I’m given a function, I like to follow a systematic approach to determine if it’s increasing or decreasing. These are the steps that I find most helpful:

A graph with a curve steadily rising from left to right, indicating an increasing function, or a curve steadily falling from left to right, indicating a decreasing function

  1. Identify the Interval: I look at the specific range of x values we’re analyzing. It’s important to know if we’re dealing with a closed or an open interval. For example, an open interval might be denoted as ((-3, 4)), meaning we don’t include the endpoints.

  2. Examine the Graph: If a graph is available, I check to see if it’s moving up or down as it progresses from left to right. Moving up implies it’s an increasing function while moving down suggests a decreasing function.

  3. Calculate the Slope: In algebra, the slope can indicate if a function is increasing or decreasing. A positive slope means the function is increasing, while a negative slope means it’s decreasing. The slope of a linear function, for example, is the coefficient of x.

  4. Derivatives for Calculus: If dealing with calculus problems, I’ll take the derivative of the function, ( f'(x) ). Then, I find where ( f'(x) = 0 ) to locate potential relative extrema points.

  5. Analyze the Average Rate of Change: The average rate of change between two points can also tell me about the behavior of the function over that interval. If it’s positive, then the function is likely increasing; if it’s negative, then it’s likely decreasing.

  6. Check for Constant Functions: If the first derivative or the slope is zero for all x-value intervals, I can conclude that the function is constant over that interval.

  7. Verify Across Intervals: Lastly, because functions can behave differently across different intervals, I make sure to check these indicators over each interval separately using interval notation, such as ([a, b]) or ((a, b)).

By applying these steps with precision and attention to detail, I avoid any content errors. It’s important to remember that functions can have different behaviors over various intervals, so I always double-check my results.

Increasing and Decreasing Functions Examples

To determine if a function is increasing or decreasing, I look at its derivative. Let me give you an example to illustrate this concept.

A graph with a curve sloping upwards to show an increasing function, and a curve sloping downwards to show a decreasing function

If I consider the function $f(x) = x^3 – 4x$, and want to find out where it is increasing or decreasing on the interval $[-1, 2]$, I need to differentiate it.

The derivative of $f(x)$ is $f'(x) = 3x^2 – 4$. To find the increasing and decreasing intervals, I set the derivative to zero to find critical points: $3x^2 – 4 = 0$. Solving this, I find that the zeros of the derivative are x = $-\sqrt{\frac{4}{3}}$ and x = $\sqrt{\frac{4}{3}}$.

Here’s a table for the sign of $f'(x)$ in each interval:

IntervalSign of DerivativeFunction Behavior
$(-\infty, -\sqrt{\frac{4}{3}})$NegativeStrictly Decreasing
$(-\sqrt{\frac{4}{3}}, \sqrt{\frac{4}{3}})$PositiveStrictly Increasing
$(\sqrt{\frac{4}{3}}, \infty)$NegativeStrictly Decreasing

For my intervals above, I considered the entire real line, but for my specific example on the interval $[-1, 2]$, I can just look at the behavior between the zeros.

Since my interval includes $-\sqrt{\frac{4}{3}}$ but not $\sqrt{\frac{4}{3}}$, I check the value of the derivative just after $-\sqrt{\frac{4}{3}}$, finding it positive, indicating the function is increasing on that segment.

To summarize, I use calculus to determine if a function is increasing or decreasing. I differentiate the function, find the zeros of the derivative within my interval, and then test the sign of the derivative around these zeros.

This allows me to identify increasing and decreasing intervals on a continuous function and further helps in understanding the function’s behavior, such as locating maximum and minimum values.


In assessing whether a function is increasing or decreasing, I’ve explained that we primarily rely on the sign of the function’s derivative.

For instance, if I take the derivative of a function, denoted as ( f'(x) ), and it yields a value greater than zero (( f'(x) > 0 )) for all ( x ) in a particular interval, then the function is increasing on that interval.

Conversely, when ( f'(x) < 0 ), the function is decreasing. It’s crucial to understand that a positive derivative indicates upward movement on a graph, while a negative derivative suggests the opposite.

In addition, critical points, where ( f'(x) = 0 ), can signal a change in the increase or decrease of a function and potentially identify local maxima or minima. However, it’s important to perform further tests, such as the First Derivative Test, to confirm such changes.

Finally, while graphical representations can offer visual confirmation of these characteristics, the analytical approach using calculus remains definitive.

My explanation aimed to clarify how derivatives serve as powerful tools in identifying the increasing and decreasing nature of functions, and I hope this has given readers confidence in analyzing functions independently.