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Conic Sections - Parabola
The parabola has the characteristic shape shown above. A parabola is defined to be the “set of points the same distance from a point and a line”.
Conic Sections - Parabola
The line is called the directrix and the point is called the focus.
Focus
Directrix
Conic Sections - Parabola
The line perpendicular to the directrix passing through the focus is the axis of symmetry. The vertex is the point of intersection of the axis of symmetry with the parabola.
Focus
Directrix
Axis of Symmetry
Vertex
Conic Sections - Parabola
The definition of the parabola is the set of points the same distance from the focus and directrix. Therefore, d1 = d2 for any point (x, y) on the parabola.
Focus
Directrix
d1
d2
Conic Sections - ParabolaThe latus rectum is the line segment passing through the focus, perpendicular to the axis of symmetry with endpoints on the parabola.
y = ax2
Focus
Vertex(0, 0)
LatusRectum
Parabola – Example 1
Graph the function. Find the vertex, axis of symmetry, focus, directrix, and latus rectum
2)2()3(8 +=− xy
Parabola – Example 2
Find the vertex, axis of symmetry, focus, directrix, endpoints of the latus rectum and sketch the graph.
0304162 2 =++−− yxy
Sample Problems
1. (y + 3)2 = 12(x -1)a. Find the vertex, axis of symmetry, focus,
directrix, and length of the latus rectum.b. Sketch the graph.
Sample Problems
2. 2x2 + 8x +3 + y = 0a. Find the vertex, focus, directrix, axis of
symmetry and length of the latus rectum.b. Sketch the graph.
Building a Table of Rules
4p(y – k) = (x – h)2
p>0 opens upp<0 opens downVertex: (h, k)Focus: (h, k + p)Directrix: y = k – pLatus Rectum: |4p|
4p(x – h) = (y – k)2
p>0 opens rightp<0 opens leftVertex: (h, k)Focus: (h + p, k)Directrix: x = h + pLatus Rectum: |4p|
Paraboloid Revolution
A paraboloid revolution results from rotating a parabola around its axis of symmetry as shown at the right.
http://commons.wikimedia.org/wiki/Image:ParaboloidOfRevolution.pngGNU Free Documentation License
Paraboloid Revolution
They are commonly used today in satellite technology as well as lighting in motor vehicle headlights and flashlights.
Paraboloid Revolution
The focus becomes an important point. As waves approach a properly positioned parabolic reflector, they reflect back toward the focus. Since the distance traveled by all of the waves is the same, the wave is concentrated at the focus where the receiver is positioned.
Example 4 – Satellite Receiver
A satellite dish has a diameter of 8 feet. The depth of the dish is 1 foot at the center of the dish. Where should the receiver be placed?
8 ft
1 ft
Let the vertex be at (0, 0). What are the coordinates of a point at the diameter of the dish?
V(0, 0)
(?, ?)