Colleen BeaudoinFor FCIT

Review: The geometric definition relies on a cone and a plane intersecting it.

Algebraic definition: All points that are equidistant from a given line (the directrix)and a fixed point not on the directrix (the focus)

Focus Directrix

x

y

Any point on the parabola is equidistant to the focus and the directrix.

Example:Point A: d1=d2

Point B: d1=d2

A

d1

d2

d1 d2

B

Vertex

Focus Directrix Axis of Symmetry

x

y

One variable is squared and one is not. (How does this differ from linear equations?)

There are many ways the equation of a parabola can be written. We will get the quadratic part (variable that is squared) on the left of the equal sign and the linear part (variable is to the first power) on the right of the equal sign.

Equation: (x - h)2 = c(y – k) OR (y - k)2 = c(x – h)

To graph:1. Put in standard form (above) – squared term

on left2. Decide which way the parabola opens.

Look at the right side. If y: + c → opens up If y: - c → opens down If x: + c → opens right If x: - c → opens left

To graph:3. Plot the vertex (h,k) Note what happens

to the signs.4. Plot the focus: move │¼ c │ from the

vertex in the direction that the parabola opens. Mark with an f.

5. Draw the directrix: │¼ c │ from the vertex in the opposite direction of the focus (Remember that the directrix is a line.)

To graph:6. Plot the endpoints of the latus

rectum/focal chord (width at the focus). The width is the │c│ at the focus.

7. Sketch the parabola by going through the vertex and the endpoints of the latus rectum. (Be sure to extend the curve and put arrows.)

8. Identify the axis of symmetry. (The line that goes through the vertex dividing the parabola in half.)

To graph:1. Put in standard form– squared term on left

Done2. Decide which way the parabola opens.

Look at the right side. If y: + c → opens up If y: - c → opens down If x: + c → opens right If x: - c → opens left

Up because y is on the right and 12 is positive

To graph:3. Plot the vertex (h,k) Note what happens to the

signs. (5,6)

4. Plot the focus: move │¼ c │ from the vertex in the direction that the parabola opens. Mark with an f. (5,9): found by moving up 3 from the vertex

5. Draw the directrix: │¼ c │ from the vertex in the opposite direction of the focus (Remember that the directrix is a line.) y = 3: found by moving down 3 from the vertex

To graph:6. Plot the endpoints of the latus rectum/focal

chord (width at the focus). The width is the │c│ at the focus. L.R. = 12 with endpoints at (-1,9) & (11,9)

7. Sketch the parabola by going through the vertex and the endpoints of the latus rectum. (Be sure to extend the curve and put arrows.)

8. Identify the axis of symmetry. (The line that goes through the vertex dividing the parabola in half.)x = 5

Vertex: (5,6)

Focus: (5,9)

Directrix: y = 3

L.R.: 12

Axis: x = 5

f

To graph:1. Put in standard form

Done2. Decide which way the parabola opens.

Left because x is on the right and 4 is negative3. Plot the vertex (h,k)

(2,-3)4. Plot the focus:

(1,-3): found by moving left 1 from the vertex5. Draw the directrix:

x = 3: found by moving right 1 from the vertex

To graph:6. Plot the endpoints of the latus rectum

L.R. = 4 with endpoints at (1,-1) & (1,-5)7. Sketch the parabola8. Identify the axis of symmetry.

y = -3

x

y Vertex: (2,-3)

Focus: (1,-3)

Directrix: x = 3

L.R.: 4

Axis: y = -3f

What’s the first step?Put in standard form. y2 - 4y + 1 = x y2 - 4y + 4 = x - 1 + 4 Complete the square. (y – 2)2 = x + 3 (y – 2)2 = 1(x + 3)

Now you try graphing the parabola and labeling all the parts.

Given the following information, write the equation of the parabola.

Vertex is (0,0) and Focus is at (0,2)

1) How can you tell the graph of an equation will be a parabola?

2) What’s the standard form of a parabola?3) What are the steps for graphing a

parabola?4) What are common errors people make

when graphing parabolas?