We've updated our
Privacy Policy effective December 15. Please read our updated Privacy Policy and tap

Study Guides > Intermediate Algebra

Read: Graph Quadratic Functions

Learning Objectives

  • Graph a quadratic function using a table of values
  • Identify important features of the graphs of a quadratic functions of the form [latex]f(x)=ax^2+bx+c[/latex]
Quadratic functions can also be graphed. It’s helpful to have an idea what the shape should be, so you can be sure that you’ve chosen enough points to plot as a guide. Let’s start with the most basic quadratic function, [latex]f(x)=x^{2}[/latex]. Graph [latex]f(x)=x^{2}[/latex]. Start with a table of values. Then think of the table as ordered pairs.
x f(x)
[latex]−2[/latex] [latex]4[/latex]
[latex]−1[/latex] [latex]1[/latex]
[latex]0[/latex] [latex]0[/latex]
[latex]1[/latex] [latex]1[/latex]
[latex]2[/latex] [latex]4[/latex]
Plot the points [latex](-2,4), (-1,1), (0,0), (1,1), (2,4)[/latex] Graph with the point negative 2, 4; the point negative 1, 1; the point 0, 0; the point 1,1; the point 2,4. Since the points are not on a line, you can’t use a straight edge. Connect the points as best you can, using a smooth curve (not a series of straight lines). You may want to find and plot additional points (such as the ones in blue here). Placing arrows on the tips of the lines implies that they continue in that direction forever. A curved U-shaped line through the points from the previous graph.   Notice that the shape is similar to the letter U. This is called a parabola. One-half of the parabola is a mirror image of the other half. The line that goes down the middle is called the line of reflection, in this case that line is they y-axis. The lowest point on this graph is called the vertex. In the following video we show an example of plotting a quadratic function using a table of values. https://youtu.be/wYfEzOJugS8 The equations for quadratic functions have the form [latex]f(x)=ax^{2}+bx+c[/latex] where [latex] a\ne 0[/latex]. In the basic graph above, [latex]a=1[/latex], [latex]b=0[/latex], and [latex]c=0[/latex]. Changing a changes the width of the parabola and whether it opens up ([latex]a>0[/latex]) or down ([latex]a<0[/latex]). If a is positive, the vertex is the lowest point, if a is negative, the vertex is the highest point. In the following example, we show how changing the value of a will affect the graph of the function.

Example

Match the following functions with their graph. a) [latex] \displaystyle f(x)=3{{x}^{2}}[/latex] b) [latex] \displaystyle f(x)=-3{{x}^{2}}[/latex] c) [latex] \displaystyle f(x)=\frac{1}{2}{{x}^{2}}[/latex] 1) compared to g(x)=x squared 2) compared to g(x)=x squared 3) compared to g(x)=x squared

Answer: Function a) [latex] \displaystyle f(x)=3{{x}^{2}}[/latex] means that inputs are squared and then multiplied by three, so the outputs will be greater than they would have been for [latex]f(x)=x^2[/latex].  This results in a parabola that has been squeezed, so graph [latex]2[/latex] is the best match for this function. compared to g(x)=x squared Function b) [latex] \displaystyle f(x)=-3{{x}^{2}}[/latex] means that inputs are squared and then multiplied by negative three, so the outputs will be farther away from the [latex]x[/latex]-axis than they would have been for [latex]f(x)=x^2[/latex], but negative in value, so graph [latex]1[/latex] is the best match for this function. compared to g(x)=x squared Function c) [latex] \displaystyle f(x)=\frac{1}{2}{{x}^{2}}[/latex] means that inputs are squared then multiplied by [latex]\dfrac{1}{2}[/latex], so the outputs are less than they would be for [latex]f(x)=x^2[/latex].  This results in a parabola that has been opened wider than[latex]f(x)=x^2[/latex]. Graph [latex]3[/latex] is the best match for this function. compared to g(x)=x squared

Answer

Function a) matches graph [latex]2[/latex] Function b) matches graph [latex]1[/latex] Function c) matches graph [latex]3[/latex]

If there is no b term, changing c moves the parabola up or down so that the y intercept is ([latex]0, c[/latex]). In the next example we show how changes to c affect the graph of the function.

Example

Match the following functions with their graph. a) [latex] \displaystyle f(x)={{x}^{2}}+3[/latex] b) [latex] \displaystyle f(x)={{x}^{2}}-3[/latex]   1) compared to g(x)=x squared 2) compared to g(x)=x squared

Answer: Function a) [latex] \displaystyle f(x)={{x}^{2}}+3[/latex] means square the inputs then add three, so every output will be moved up [latex]3[/latex] units. The graph that matches this function best is [latex]2[/latex]. compared to g(x)=x squared Function b) [latex] \displaystyle f(x)={{x}^{2}}-3[/latex]  means square the inputs then subtract three, so every output will be moved down [latex]3[/latex] units. The graph that matches this function best is [latex]1[/latex]. compared to g(x)=x squared

Changing [latex]b[/latex] moves the line of reflection, which is the vertical line that passes through the vertex ( the high or low point) of the parabola. It may help to know how calculate the vertex of a parabola to understand how changing the value of [latex]b[/latex] in a function will change its graph. To find the vertex of the parabola, use the formula [latex] \displaystyle \left( \frac{-b}{2a},f\left( \frac{-b}{2a} \right) \right)[/latex]. For example, if the function in consideration is [latex]f(x)=2x^2-3x+4[/latex], to find the vertex, first calculate [latex]\Large\frac{-b}{2a}[/latex] [latex]a = 2[/latex], and [latex]b = -3[/latex], therefore [latex]\dfrac{-b}{2a}=\dfrac{-(-3)}{2(2)}=\dfrac{3}{4}[/latex]. This is the [latex]x[/latex] value of the vertex. Now evaluate the function at [latex]x =\Large\frac{3}{4}[/latex] to get the corresponding y-value for the vertex. [latex]f\left( \dfrac{-b}{2a} \right)=2\left(\dfrac{3}{4}\right)^2-3\left(\dfrac{3}{4}\right)+4=2\left(\dfrac{9}{16}\right)-\dfrac{9}{4}+4=\dfrac{23}{8}[/latex]. The vertex is at the point [latex]\left(\dfrac{3}{4},\dfrac{23}{8}\right)[/latex].  This means that the vertical line of reflection passes through this point as well.  It is not easy to tell how changing the values for [latex]b[/latex] will change the graph of a quadratic function, but if you find the vertex, you can tell how the graph will change. In the next example we show how changing b can change the graph of the quadratic function.

Example

Match the following functions with their graph. a) [latex] \displaystyle f(x)={{x}^{2}}+2x[/latex] b) [latex] \displaystyle f(x)={{x}^{2}}-2x[/latex] a) compared to g(x)=x squared b) compared to g(x)=x squared

Answer: Find the vertex of function a)[latex] \displaystyle f(x)={{x}^{2}}+2x[/latex]. [latex-display]a = 1, b = 2[/latex-display] x-value: [latex-display]\dfrac{-b}{2a}=\dfrac{-2}{2(1)}=-1[/latex-display] y-value: [latex]f(\dfrac{-b}{2a})=(-1)^2+2(-1)=1-2=-1[/latex]. Vertex = [latex](-1,-1)[/latex], which means the graph that best fits this function is a) compared to g(x)=x squared   Find the vertex of function b) [latex]f(x)={{x}^{2}}-2x[/latex]. [latex-display]a = 1, b = -2[/latex-display] x-value: [latex-display]\dfrac{-b}{2a}=\dfrac{2}{2(1)}=1[/latex-display] y-value: [latex]f(\dfrac{-b}{2a})=(1)^2-2(1)=1-2=-1[/latex]. Vertex = [latex](1,-1)[/latex], which means the graph that best fits this function is b) compared to g(x)=x squared

Note that the vertex can change if the value for c changes because the y-value of the vertex is calculated by substituting the x-value into the function. Here is a summary of how the changes to the values for a, b, and, c of a quadratic function can change it's graph.

Properties of a Parabola

For [latex] \displaystyle f(x)=a{{x}^{2}}+bx+c[/latex], where a, b, and c are real numbers.
  • The parabola opens upward if [latex]a > 0[/latex] and downward if [latex]a < 0[/latex].
  • a changes the width of the parabola. The parabola gets narrower if [latex]|a|> 1[/latex] and wider if [latex]|a|<1[/latex].
  • The vertex depends on the values of a, b, and c. The vertex is [latex]\left(\dfrac{-b}{2a},f\left( \dfrac{-b}{2a}\right)\right)[/latex].
In the last example we show how you can use the properties of a parabola to help you make a graph without having to calculate an exhaustive table of values.

Example

Graph [latex]f(x)=−2x^{2}+3x–3[/latex].

Answer: Before making a table of values, look at the values of a and c to get a general idea of what the graph should look like. [latex]a=−2[/latex], so the graph will open down and be thinner than [latex]f(x)=x^{2}[/latex]. [latex]c=−3[/latex], so it will move to intercept the y-axis at [latex](0,−3)[/latex]. To find the vertex of the parabola, use the formula [latex] \displaystyle \left( \dfrac{-b}{2a},f\left( \dfrac{-b}{2a} \right) \right)[/latex]. Finding the vertex may make graphing the parabola easier.

[latex]\text{Vertex }\text{formula}=\left( \dfrac{-b}{2a},f\left( \dfrac{-b}{2a} \right) \right)[/latex]

x-coordinate of vertex:

[latex] \displaystyle \dfrac{-b}{2a}=\dfrac{-(3)}{2(-2)}=\dfrac{-3}{-4}=\dfrac{3}{4}[/latex]

y-coordinate of vertex:

[latex] \displaystyle \begin{array}{l}f\left( \dfrac{-b}{2a} \right)=f\left( \dfrac{3}{4} \right)\\\,\,\,f\left( \dfrac{3}{4} \right)=-2{{\left( \dfrac{3}{4} \right)}^{2}}+3\left( \dfrac{3}{4} \right)-3\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,=-2\left( \dfrac{9}{16} \right)+\dfrac{9}{4}-3\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,=\dfrac{-18}{16}+\dfrac{9}{4}-3\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,=\dfrac{-9}{8}+\dfrac{18}{8}-\dfrac{24}{8}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,=-\dfrac{15}{8}\end{array}[/latex]

Vertex: [latex] \displaystyle \left( \dfrac{3}{4},-\dfrac{15}{8} \right)[/latex] Use the vertex, [latex] \displaystyle \left( \dfrac{3}{4},-\dfrac{15}{8} \right)[/latex], and the properties you described to get a general idea of the shape of the graph. You can create a table of values to verify your graph. Notice that in this table, the x values increase. The y values increase and then start to decrease again. That indicates a parabola.
x f(x)
[latex]−2[/latex] [latex]−17[/latex]
[latex]−1[/latex] [latex]−8[/latex]
[latex]0[/latex] [latex]−3[/latex]
[latex]1[/latex] [latex]−2[/latex]
[latex]2[/latex] [latex]−5[/latex]
Vertex at negative three-fourths, negative 15-eighths. Other points are plotted: the point negative 2, negative 17; the point negative 1, negative 8; the point 0, negative 3; the point 1, negative 2; and the point 2, negative 5.

Answer

A parabola drawn through the points in the previous graph Connect the points as best you can, using a smooth curve. Remember that the parabola is two mirror images, so if your points don’t have pairs with the same value, you may want to include additional points (such as the ones in blue here). Plot points on either side of the vertex. [latex]x=\Large\frac{1}{2}[/latex] and [latex]x=\Large\frac{3}{2}[/latex] are good values to include.

The following video shows another example of plotting a quadratic function using the vertex. https://youtu.be/leYhH_-3rVo Creating a graph of a function is one way to understand the relationship between the inputs and outputs of that function. Creating a graph can be done by choosing values for x, finding the corresponding y values, and plotting them. However, it helps to understand the basic shape of the function. Knowing the effect of changes to the basic function equation is also helpful. One common shape you will see is a parabola. Parabolas have the equation [latex]f(x)=ax^{2}+bx+c[/latex], where a, b, and c are real numbers and [latex]a\ne0[/latex]. The value of a determines the width and the direction of the parabola, while the vertex depends on the values of a, b, and c. The vertex is [latex] \displaystyle \left( \dfrac{-b}{2a},f\left( \dfrac{-b}{2a} \right) \right)[/latex].

Licenses & Attributions

CC licensed content, Original

  • Graph a Quadratic Function Using a Table of Value and the Vertex. Authored by: James Sousa (Mathispower4u.com) for Lumen Learning. License: CC BY: Attribution.
  • Revision and Adaptation. Provided by: Lumen Learning License: CC BY: Attribution.

CC licensed content, Shared previously

  • Unit 17: Functions, from Developmental Math: An Open Program. Provided by: Monterey Institute of Technology and Education Located at: https://www.nroc.org/. License: CC BY: Attribution.
  • Ex: Graph a Quadratic Function Using a Table of Values. Authored by: James Sousa (Mathispower4u.com) for Lumen Learning. License: CC BY: Attribution.