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A traditional view of the conics is included.
Pick a point in space to use as a pivot point and vertex. Pass a line through the point and rotate it tracing two cones. Each cone is called a nape. Next take a plane and think of the ways the place may cut the and cones. Lines, parabolas, circles, ellipses, hyperbolas, and points are all conic sections, or curves which are formed when one or both napes are cut by the plane.
For our purposes, conics must be written as the union of two functions. The traditional conic section equations, must be rewritten as expressions for y. In each case, one manipulation of the equation looks like "y² = ..." or "(y  k)² = ..." When the square root of each side is taken, two seperated functions for y result, one is negative one is positive.
The remainer of the page is meant only as examples, not as an exhaustive list of important points.
The Squaring and Square Root Functions from Function and Relation Library. The Quadratic, often written as y = ax² + bx + c or y = a(x  h)² + k, is a parabola.

The circle x² + y² = r² having a radius of r and centered at (0,0) may be graphed as the union of the two functions
y = (r²  x²) and y =  (r²  x²).
On Circles 

The circle x² + y² = 3² having a radius of 3 and centered at (0,0) may be graphed as the union of the two functions
y = (3²  x²) and y =  (3²  x²).
The circle (x  h)² + (y  k)² = r² having a radius of r and a center at (h,k) must be graphed as the union of functions:
Examples follow.
Circles with: center at (1,3) and radius of 4 center at (2,1) and radius of 1 center at (4,4) and radius of 2 
Circle or Ellipse on SketchPad
The standard equation for the ellipse,
also requires two function equations:
The center is (h,k) and 2a and 2b are the lengths of the major and minor axes.
Circle or Ellipse on SketchPad
The standard equation for the hyperbola is like that of the ellipse.
It requires two functions.
At left,
asymptotes
are graphed as well as the hyperbola.
y=f(x) Hyperbola or x=f(y) Hyperbola on SketchPad
This page is from Exploring Functions Throught the Use of Manipulatives (ISBN: 0962359335).
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