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Parabolas: Definition, Focus, Directrix, and Graphing, Lecture notes of Mathematics

The concept of parabolas as conic sections, introducing the definitions of focus, directrix, and vertex. It provides equations and examples for graphing vertical and horizontal parabolas with and without the vertex at the origin.

Typology: Lecture notes

2021/2022

Uploaded on 09/12/2022

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Math 1330 – Section 8.1
Parabolas
Next, we’ll look at parabolas. We previously studied parabolas as the graphs of quadratic
functions. Now we will look at them as conic sections. There are a few differences. For
example, when we studied quadratic functions, we saw that the graphs of the functions
could open up or down. As we look at conic sections, we’ll see that the graphs of these
second degree equations can also open left or right. So, not every parabola we’ll look at
in this section will be a function.
We already know that the graph of a quadratic function
f x
ax bx
c( ) = + +
2
is a
parabola. But there is more to be learned about parabolas.
Definition: A parabola is the set of all points equally distant from a fixed line and a
fixed point not on the line. The fixed line is called the directrix. The fixed point is called
the focus.
The axis, or axis of symmetry, runs through the focus and is perpendicular to the directrix.
The vertex is the point halfway between the focus and the directrix.
We won’t be working with slanted parabolas, just with “horizontal” and “vertical”
parabolas.
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Math 1330 – Section 8. Parabolas

Next, we’ll look at parabolas. We previously studied parabolas as the graphs of quadratic functions. Now we will look at them as conic sections. There are a few differences. For example, when we studied quadratic functions, we saw that the graphs of the functions could open up or down. As we look at conic sections, we’ll see that the graphs of these second degree equations can also open left or right. So, not every parabola we’ll look at in this section will be a function.

We already know that the graph of a quadratic function f ( ) x = ax^2 + bx + c is a

parabola. But there is more to be learned about parabolas.

Definition: A parabola is the set of all points equally distant from a fixed line and a fixed point not on the line. The fixed line is called the directrix. The fixed point is called the focus.

The axis , or axis of symmetry , runs through the focus and is perpendicular to the directrix.

The vertex is the point halfway between the focus and the directrix.

We won’t be working with slanted parabolas, just with “horizontal” and “vertical” parabolas.

- p p

Basic “Horizontal” Parabola:

Equation: y^2 = 4 px

Focus: ( p , 0)

Directrix: x = − p

Focal Width: 4 p

Note: This is not a function (fails vertical line test). However, the top half y = x is a

function and the bottom half y = − x is also a function.

Graphing parabolas with vertex not at the origin:

  • Rearrange (complete the square) to look like ( yk )^2 = 4 p x ( − h )or ( xh )^2 = 4 p y ( − k ).
  • Vertex is ( , h k ). Draw it the same way, except start at this vertex.

Example 1: Write in standard form and graph it.

Vertex:

Focus:

Directrix:

Focal width:

Endpoints of focal chord:

Example 3: Write in standard form and graph it.

Vertex:

Focus:

Directrix:

Focal width:

Endpoints of focal chord:

Example 4: Suppose you know that the vertex of a parabola is at (-3, 5) and its focus is at (1, 5). Write an equation for the parabola in standard form.

Example 5: Suppose you know that the focus of a parabola is (-1, 3) and the directrix is the line y =− 1. Write an equation for the parabola in standard form.