What can calculus tell us about curve sketching? It turns out, quite a lot! In this article, you’ll see a list of the 10 key characteristics that describe a graph. While you may not be tested on your artistic ability to sketch a curve on the AP Calculus exams, you *will* be expected to determine these specific features of graphs.

## Guide to Curve Sketching

The ten steps of curve sketching each require a specific tool. But some of the steps are closely related. In the list below, you’ll see some steps grouped if they are based on similar methods.

- Domain and Range
*y*-Intercept*x*-Intercept(s)- Symmetry
- Vertical Asymptote(s)
- Horizontal and/or Oblique Asymptote(s)
- Increase/Decrease
- Relative Extrema
- Concavity
- Inflection Points

**Algebra and pre-calculus**

**Limits**

**First Derivative**

**Second Derivative**

Some books outline these steps differently, sometimes combining items together. So it’s not uncommon to see “The Eight Steps for Curve Sketching,” etc.

Let’s briefly review what each term means. More details can be found at AP Calculus Exam Review: Analysis of Graphs, for example.

### Step 1. Determine the Domain and Range

The **domain** of a function *f*(*x*) is the set of all input values (*x*-values) for the function.

The **range** of a function *f*(*x*) is the set of all output values (*y*-values) for the function.

Methods for finding the domain and range vary from problem to problem. Here is a good review.

### Step 2. Find the *y*-Intercept

The ** y-intercept** of a function

*f*(

*x*) is the point where the graph crosses the

*y*-axis.

This is easy to find. Simply plug in 0. The *y*-intercept is: (0, *f*(*0*)).

### Step 3. Find the *x*-Intercept(s)

An ** x-intercept** of a function

*f*(

*x*) is any point where the graph crosses the

*x*-axis.

To find the *x*-intercepts, solve *f*(*x*) = 0.

### Step 4. Look for Symmetry

A graph can display various kinds of symmetry. Three main symmetries are especially important: *even*, *odd*, and *periodic* symmetry.

**Even symmetry.**A function is**even**if its graph is symmetric by reflection over the*y*-axis.**Odd symmetry.**A function is**odd**if its graph is symmetric by 180 degree rotation around the origin.**Periodicity.**A function is**periodic**if an only if its values repeat regularly. That is, if there is a value*p*> 0 such that*f*(*x*+*p*) =*f*(*x*) for all*x*in its domain.

The algebraic test for even/odd is to plug in (-*x*) into the function.

- If
*f*(-*x*) =*f*(*x*), then*f*is even. - If
*f*(-*x*) = –*f*(*x*), then*f*is odd.

On the AP Calculus exams, periodicity occurs only in trigonometric functions.

### Step 5. Find any Vertical Asymptote(s)

A **vertical asymptote** for a function is a vertical line *x* = *k* showing where the function becomes unbounded.

For details, check out How do you find the Vertical Asymptotes of a Function?.

### Step 6. Find Horizontal and/or Oblique Asymptote(s)

A **horizontal asymptote** for a function is a horizontal line that the graph of the function approaches as *x* approaches ∞ or -∞.

An **oblique asymptote** for a function is a slanted line that the function approaches as *x* approaches ∞ or -∞.

Both horizontal and oblique asymptotes measure the *end behavior* of a function. For details, see How do you find the Horizontal Asymptotes of a Function? and How do you find the Oblique Asymptotes of a Function?.

### Step 7. Determine the Intervals of Increase and Decrease

A function is **increasing** on an interval if the graph rises as you trace it from left to right.

A function is **decreasing** on an interval if the graph falls as you trace it from left to right.

The first derivative measures increase/decrease in the following way:

- If
*f*'(*x*) > 0 on an interval, then*f*is increasing on that interval. - If
*f*'(*x*) < 0 on an interval, then*f*is decreasing on that interval.

### Step 8. Locate the Relative Extrema

The term **relative extrema** refers to both relative minimum and relative maximum points on a graph.

A graph has a **relative maximum** at *x* = *c* if *f*(*c*) > *f*(*x*) for all *x* in a small enough neighborhood of *c*.

A graph has a **relative minimum** at *x* = *c* if *f*(*c*) < *f*(*x*) for all *x* in a small enough neighborhood of *c*.

The relative maxima *(plural of maximum)* and minima *(plural of minimum)* are the “peaks and valleys” of the graph. There can be many relative maxima and minima in any given graph.

Relative extrema occur at points where *f* '(*x*) = 0 or *f* '(*x*) does not exist. Use the First Derivative Test to classify them.

### Step 9. Determine the Intervals of Concavity

Concavity is a measure of how curved the graph of the function is at various points. For example, a *linear* function has zero concavity at all points, because a line simply does not curve.

A graph is **concave up** on an interval if the tangent line falls below the curve at each point in the interval. In other words, the graph curves “upward,” away from its tangent lines.

A graph is **concave down** on an interval if the tangent line falls above the curve at each point in the interval. In other words, the graph curves “downward,” away from its tangent lines.

Here’s one way to remember the definitions: “Concave up looks like a cup, and concave down looks like a frown.”

The second derivative measures concavity:

- If
*f*''(*x*) > 0 on an interval, then*f*is concave up on that interval. - If
*f*''(*x*) < 0 on an interval, then*f*is concave down on that interval.

### Step 10. Locate the Inflection Points

Any point at which concavity changes (from up to down or down to up) is called a **point of inflection**.

Any point where *f* ''(*x*) = 0 or *f* ''(*x*) does not exist is a possible point of inflection. Look for changes in concavity to determine if these are actual points of inflection.

## Final Thoughts

This short article only outlines the steps for accurate curve sketching. Now it’s up to you to familiarize yourself with the various methods and tools that will help you to analyze the graph of any function.

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##### About Shaun Ault

Shaun earned his Ph. D. in mathematics from The Ohio State University in 2008 (Go Bucks!!). He received his BA in Mathematics with a minor in computer science from Oberlin College in 2002. In addition, Shaun earned a B. Mus. from the Oberlin Conservatory in the same year, with a major in music composition. Shaun still loves music -- almost as much as math! -- and he (thinks he) can play piano, guitar, and bass. Shaun has taught and tutored students in mathematics for about a decade, and hopes his experience can help you to succeed!

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