Introduction to Quadratic Functions
A. Definition
is
a quadratic function if
where a≠0
The graph is a parabola
B. Parabolas
1. Parabolas
2. v is the vertex of the parabola
C. Formula for the Vertex
1. Vertex Formula: | ![]() |
2. See MTH103: College Algebra for the justification of this formula
D. Graphing Quadratic Functions (Parabolas)
1. Locate the vertex: use the vertex formula
2. Find the x and y intercepts.
3. Plot the points and connect in a smooth curve, recognizing whether it opens
up or down.
E. Examples of Graphing
Example 1: Graphwhere
Solution
1. Locate vertex:
Vertex Formula:
Thus the vertex is (-3,-3)
2. Intercepts
y-intercept:
x-intercept:
3. Note the graph opens upward:
Example 2: Graphwhere
Solution
1. Locate vertex:
Vertex Formula:
Thus the vertex is (1,-5)
2. Intercepts
y-intercept:
x-intercept:
no real solutions!
Thus, no x-intercept
3. Note the graph opens downward:
F. Projectile Motion
A thrown object follows a parabolic path given by
initial upward speed
initial height thrown
from
Units of height: feet
The vertex is the peak of the path.
Features
1. The t-coordinate (“ x-coordinate”) of the vertex tells
you
when the projectile reaches its maximum height
2. The “y-coordinate” of the vertex tells you what the maximum height is.
3. The projectile hits the ground when
G. Projectile Motion Example
A ball is thrown upward with a speed of 16ft/s from a
building 32 feet high.
What is the maximum height of the ball? When does it reach the ground?
Solution
Use the projectile motion model:
Now
Also,
Thus the vertex is (1/2,36)
Now the maximum height is the y-coordinate of the vertex: 36 feet
The ball reaches the ground when
Since t=1 is not physical, we must have t=2 .
Thus the ball reaches the ground 2 seconds after being thrown.