1. 9.1 Graphing Quadratic Functions

2. Quadratic Function
A function of the form y=ax2+bx+c where a≠0 making a u-shaped graph called a parabola.
- If a is positive, u opens up
- If a is negative, u opens down

3. Vertex: The lowest point (minimum) or highest point (maximum)
of a parabola
Axis of symmetry:
The vertical line through the vertex of the parabola.
Vertex
Axis of
Symmetry

4. Ex 1:
Vertex:
Axis of symmetry:
Domain:
Range:

5. Standard Form Equation: y= ax2 + bx + c

6. Find the Axis of Symmetry. (the vertical line x= ) Find the Vertex
Steps for Graphing
Find the Axis of Symmetry. (the vertical line x= )
Find the Vertex
the x-coordinate of the vertex is
plug x-coordinate into the equation to find y-coordinate
Find the Y-intercept
plug 0 in for x to get y
Yint: (0, c)

7. Ex 2: y = –x2 + 5x – 2 Axis of symmetry: Vertex: Y-intercept: Domain:
Range:

8. Example: y= –x2 + 5x – 2

9. 9.1 Graphing Quadratic Functions
Homework:
pg #5, 7, 9, 11, 17 (5 problems)

10. Vertex Form Equation y=a(x-h)2+k If a is positive, parabola opens up
If a is negative, parabola opens down.
The vertex is the point (h,k).
The axis of symmetry is the vertical line x=h.
Don’t forget about 2 points on either side of the vertex! (5 points total!)

11. Intercept Form Equation
y=a(x-p)(x-q)
The x-intercepts are the points (p,0) and (q,0).
The axis of symmetry is the vertical line x=
The x-coordinate of the vertex is
To find the y-coordinate of the vertex, plug the x-coord. into the equation and solve for y.
If a is positive, parabola opens up
If a is negative, parabola opens down.

12. Example 1: Graph y=2x2-8x+6 Axis of symmetry is the vertical line x=2
Table of values for other points: x y
0 6
1 0
2 -2
3 0
4 6
* Graph!
a=2 Since a is positive the parabola will open up.
Vertex: use
b=-8 and a=2
Vertex is: (2,-2)
x=2

13. Now you try one. y=-x2+x+12. Open up or down. Vertex. Axis of symmetry
Now you try one! y=-x2+x+12 * Open up or down? * Vertex? * Axis of symmetry? * Table of values with 5 points?

14. (.5,12)
(-1,10)
(2,10)
(-2,6)
(3,6)
X = .5

15. Example 2: Graph y=-.5(x+3)2+4
a is negative (a = -.5), so parabola opens down.
Vertex is (h,k) or (-3,4)
Axis of symmetry is the vertical line x = -3
Table of values x y
-1 2
-3 4
-5 2
Vertex (-3,4)
(-4,3.5)
(-2,3.5)
(-5,2)
(-1,2)
x=-3

16. Table of values with 5 points?
Now you try one!
y=2(x-1)2+3
Open up or down?
Vertex?
Axis of symmetry?
Table of values with 5 points?

17. (-1, 11)
(3,11)
X = 1
(0,5)
(2,5)
(1,3)

18. Example 3: Graph y=-(x+2)(x-4)
Since a is negative, parabola opens down.
The x-intercepts are (-2,0) and (4,0)
To find the x-coord. of the vertex, use
To find the y-coord., plug 1 in for x.
Vertex (1,9)
The axis of symmetry is the vertical line x=1 (from the x-coord. of the vertex)
(1,9)
(-2,0)
(4,0)
x=1

19. Now you try one! y=2(x-3)(x+1) Open up or down? X-intercepts? Vertex?
Axis of symmetry?

20. x=1
(-1,0)
(3,0)
(1,-8)

21. Changing from vertex or intercepts form to standard form
The key is to FOIL! (first, outside, inside, last)
Ex: y=-(x+4)(x-9) Ex: y=3(x-1)2+8
=-(x2-9x+4x-36) =3(x-1)(x-1)+8
=-(x2-5x-36) =3(x2-x-x+1)+8
y=-x2+5x =3(x2-2x+1)+8
=3x2-6x+3+8
y=3x2-6x+11

22. Assignment