1. Introduction
We have studied the key features of the graph of a parabola, such as the vertex and x-intercepts. In this lesson, we will review the definitions of key features and learn to identify them from the equation of a quadratic function. Once you identify the key features, you can make a sketch of the graph of a quadratic equation.
5.6.1: Graphing Quadratic Functions

2. Key Concepts
A quadratic function in standard form, or general form, is written as f(x) = ax2 + bx + c, where a is the coefficient of the quadratic term, b is the coefficient of the linear term, and c is the constant term.
The graph of a quadratic is U-shaped and called a parabola.
The extremum of a graph is the function value that achieves either a maximum or a minimum.
The maximum is the largest y-value of a quadratic, and the minimum is the smallest y-value of a quadratic.
5.6.1: Graphing Quadratic Functions

3. Key Concepts, continued
If a > 0, the parabola is concave up and the quadratic has a minimum.
If a < 0, the parabola is concave down and the quadratic has a maximum.
The extreme values of a quadratic occur at the vertex, the point at which the curve changes direction.
If the quadratic equation is given in standard form, the vertex can be found by identifying the x-coordinate of the vertex using
5.6.1: Graphing Quadratic Functions

4. Key Concepts, continued
Substitute the value of the x-coordinate into the quadratic equation to find the y-coordinate of the vertex.
The vertex of a quadratic function is
Another way to find the vertex is to complete the square on the standard form of the parabola in order to convert it to vertex form.
The vertex form of a quadratic function is f(x) =
a(x – h)2 + k, where the coordinate pair (h, k) is the location of the vertex.
5.6.1: Graphing Quadratic Functions

5. Key Concepts, continued
The intercept of a graph is the point at which a line intercepts the x- or y-axis.
The y-intercept of a function is the point at which the graph crosses the y-axis. This occurs when x = 0. The y-intercept is written as (0, y).
The x-intercepts of a function are the points at which the graph crosses the x-axis. This occurs when y = 0. The x-intercept is written as (x, 0).
5.6.1: Graphing Quadratic Functions

6. Key Concepts, continued
The zeros of a function are the x-values for which the function value is 0.
The intercept form of the quadratic function, written as f(x) = a(x – p)(x – q), where p and q are the zeros of the function, can be used to identify the x-intercepts. Set the factored form equal to 0. Then set each factor equal to 0. As long as the coefficients of x are 1, the x-intercepts are located at (r, 0) and (s, 0).
These values for x are the roots, or the solutions, of the quadratic equation.
5.6.1: Graphing Quadratic Functions

7. Key Concepts, continued
Parabolas are symmetrical; that is, they have two identical parts when rotated around a point or reflected over a line.
This line is the axis of symmetry, the line through the vertex of a parabola about which the parabola is symmetric. The equation of the axis of symmetry is
Symmetry can be used to find the vertex of a parabola if the vertex is not known.
5.6.1: Graphing Quadratic Functions

8. Key Concepts, continued
If you know the x-intercepts of the graph or any two points on the graph with the same y-value, the x-coordinate of the vertex is the point halfway between the values of the x-coordinates.
For x-intercepts (r, 0) and (s, 0), the x-coordinate of the vertex is
5.6.1: Graphing Quadratic Functions

9. Key Concepts, continued
5.6.1: Graphing Quadratic Functions

10. Key Concepts, continued
To graph a function using a graphing calculator, follow these general steps for your calculator model.
On a TI-83/84:
Step 1: Press the [Y=] button.
Step 2: Type the function into Y1, or any available equation. Use the [X, T, θ, n] button for the variable x. Use the [x2] button for a square.
Step 3: Press [WINDOW]. Enter values for Xmin, Xmax, Ymin, and Ymax. The Xscl and Yscl are arbitrary. Leave Xres = 1.
Step 4: Press [GRAPH].
5.6.1: Graphing Quadratic Functions

11. Key Concepts, continued
On a TI-Nspire:
Step 1: Press the [home] key.
Step 2: Arrow over to the graphing icon and press [enter].
Step 3: Type the function next to f1(x), or any available equation, and press [enter]. Use the [X] button for the letter x. Use the [x2] button for a square.
Step 4: To change the viewing window, press [menu]. Select 4: Window/Zoom and select A: Zoom – Fit.
5.6.1: Graphing Quadratic Functions

12. Common Errors/Misconceptions
incorrectly replacing x with 0 instead of y when determining the x-intercept (and vice versa)
using the incorrect sign for the x-coordinate of the vertex when using vertex form
using the incorrect sign when identifying the intercepts from the factored form of the quadratic
5.6.1: Graphing Quadratic Functions

13. Guided Practice Example 1
Given the function f(x) = –2x2 + 16x – 30, identify the key features of the graph: the extremum, vertex, x-intercept(s), and y-intercept. Then sketch the graph.
5.6.1: Graphing Quadratic Functions

14. Guided Practice: Example 1, continued
Determine the extremum of the graph.
The extreme value is a minimum when a > 0. It is a maximum when a < 0.
Because a = –2, the graph opens downward and the quadratic has a maximum.
5.6.1: Graphing Quadratic Functions

15. Guided Practice: Example 1, continued
Determine the vertex of the graph.
The maximum value occurs at the vertex.
The vertex is of the form
Use the original equation f(x) = –2x2 + 16x – 30 to find the values of a and b in order to find the x-value of the vertex.
5.6.1: Graphing Quadratic Functions

16. Guided Practice: Example 1, continued
The x-coordinate of the vertex is 4.
Formula to find the
x-coordinate of the vertex of a quadratic
Substitute –2 for a and 16 for b.
x = 4
Simplify.
5.6.1: Graphing Quadratic Functions

17. Guided Practice: Example 1, continued
Substitute 4 into the original equation to find the y-coordinate.
The y-coordinate of the vertex is 2.
The vertex is located at (4, 2).
f(x) = –2x2 + 16x – 30
Original equation
f(4) = –2(4)2 + 16(4) – 30
Substitute 4 for x.
f(4) = 2
Simplify.
5.6.1: Graphing Quadratic Functions

18. Guided Practice: Example 1, continued
Determine the x-intercept(s) of the graph.
Since the vertex is above the x-axis and the graph opens downward, there will be two x-intercepts.
Factor the quadratic and set each factor equal to 0.
5.6.1: Graphing Quadratic Functions

19. Guided Practice: Example 1, continued
The x-intercepts are (3, 0) and (5, 0).
f(x) = –2x2 + 16x – 30
Original equation
f(x) = –2(x2 – 8x + 15)
Factor out the greatest common factor.
f(x) = –2(x – 3)(x – 5)
Factor the trinomial.
0 = –2(x – 3)(x – 5)
Set the factored form equal to 0 to find the intercepts.
x – 3 = 0 or x – 5 = 0
Set each factor equal to 0 and solve for x.
x = 3 or x = 5
Simplify.
5.6.1: Graphing Quadratic Functions

20. Guided Practice: Example 1, continued
Determine the y-intercept of the graph.
The y-intercept occurs when x = 0.
Substitute 0 for x in the original equation.
The y-intercept is (0, –30).
When the quadratic equation is written in standard form, the y-intercept is c.
f(x) = –2x2 + 16x – 30
Original equation
f(0) = –2(0)2 + 16(0) – 30
Substitute 0 for x.
f(0) = –30
Simplify.
5.6.1: Graphing Quadratic Functions

21. Guided Practice: Example 1, continued Graph the function.
Use symmetry to identify additional points on the graph.
The axis of symmetry goes through the vertex, so the axis of symmetry is x = 4.
For each point to the left of the axis of symmetry, there is another point the same distance on the right side of the axis and vice versa.
The point (0, –30) is on the graph, and 0 is 4 units to the left of the axis of symmetry.
5.6.1: Graphing Quadratic Functions

22. Guided Practice: Example 1, continued
The point that is 4 units to the right of the axis is 8, so the point (8, –30) is also on the graph.
Determine two additional points on the graph.
Choose an x-value to the left or right of the vertex and find the corresponding y-value.
f(x) = –2x2 + 16x – 30
Original equation
f(1) = –2(1)2 + 16(1) – 30
Substitute 1 for x.
f(1) = –16
Simplify.
5.6.1: Graphing Quadratic Functions

23. Guided Practice: Example 1, continued
An additional point is (1, –16).
(1, –16) is 3 units to the left of the axis of symmetry.
The point that is 3 units to the right of the axis is 7, so the point (7, –16) is also on the graph.
Plot the points and join them with a smooth curve.
5.6.1: Graphing Quadratic Functions

24. Guided Practice: Example 1, continued✔
5.6.1: Graphing Quadratic Functions

25. Guided Practice: Example 1, continued
5.6.1: Graphing Quadratic Functions

26. Guided Practice Example 3
Given the function f(x) = –2(x + 1)(x + 5), identify the key features of its graph: the extremum, vertex, x-intercept(s), and y-intercept. Then sketch the graph.
5.6.1: Graphing Quadratic Functions

27. Guided Practice: Example 3, continued
Determine the extremum of the graph.
The extreme value is either a minimum, when a > 0, or a maximum, when a < 0.
Because a = –2, the graph opens down and the quadratic has a maximum.
5.6.1: Graphing Quadratic Functions

28. Guided Practice: Example 3, continued
Determine the x-intercept(s) of the graph.
The equation is in factored form.
The zeros occur when y = 0.
Find the zeros by setting the equation equal to 0.
Then set each factor equal to 0.
5.6.1: Graphing Quadratic Functions

29. Guided Practice: Example 3, continued
The x-intercepts are (–1, 0) and (–5, 0).
f(x) = –2(x + 1)(x + 5)
Original equation
0 = –2(x + 1)(x + 5)
Set the equation equal to 0.
x + 1 = 0 or x + 5 = 0
x = –1 or x = –5
5.6.1: Graphing Quadratic Functions

30. Guided Practice: Example 3, continued
Determine the vertex of the graph.
Use the axis of symmetry to identify the vertex.
The axis of symmetry is halfway between the
x-intercepts.
Find the midpoint between the x-intercepts.
5.6.1: Graphing Quadratic Functions

31. Guided Practice: Example 3, continued
The axis of symmetry is x = –3, so the x-coordinate of the vertex is –3.
To find the y-coordinate, substitute –3 into the original equation.
Midpoint formula
Substitute –1 for x1 and –5 for x2.
Simplify.
5.6.1: Graphing Quadratic Functions

32. Guided Practice: Example 3, continued The vertex is (–3, 8).
f(x) = –2(x + 1)(x + 5)
Original equation
f(–3) = –2[(–3) + 1][(–3) + 5]
Substitute –3 for x.
f(–3) = –2(–2)(2)
Simplify.
f(–3) = 8
5.6.1: Graphing Quadratic Functions

33. Guided Practice: Example 3, continued
Determine the y-intercept of the graph.
The y-intercept occurs when x = 0.
Substitute 0 for x in the original equation.
The y-intercept is (0, –10).
f(x) = –2(x + 1)(x + 5)
Original equation
f(0) = –2[(0) + 1][(0) + 5]
Substitute 0 for x.
f(0) = –2(1)(5)
Simplify.
f(0) = –10
5.6.1: Graphing Quadratic Functions

34. Guided Practice: Example 3, continued Graph the function.
Use symmetry to identify additional points on the graph.
Because 0 is 3 units to the right of the axis of symmetry, the point 3 units to the left of the axis will have the same value, so (–6, –10) is also on the graph.
Determine two additional points on the graph.
Choose an x-value to the left or right of the vertex and find the corresponding y-value.
5.6.1: Graphing Quadratic Functions

35. Guided Practice: Example 3, continued
An additional point is (–2, 6).
(–2, 6) is 1 unit to right of the axis of symmetry.
The point that is 1 unit to the left of the axis is –4, so the point (–4, 6) is also on the graph.
Plot the points and join them with a smooth curve.
f(x) = –2(x + 1)(x + 5)
Original equation
f(–2) = –2[(–2) + 1][(–2) + 5]
Substitute –2 for x.
f(–2) = 6
Simplify.
5.6.1: Graphing Quadratic Functions

36. Guided Practice: Example 3, continued✔
5.6.1: Graphing Quadratic Functions

37. Guided Practice: Example 3, continued
5.6.1: Graphing Quadratic Functions