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on June 10, 2007 at 11:04:58 am

### E3 (adv) - write the equations of circles and ellipses in transformational form and as mapping rules to visualize and sketch graphs.

Lindsay

Definitions:

Circle: a single, closed curved line

Ellipse: a single, closed curved line that has one axis longer than the other (x-axis or y-axis) *Example below has a longer y-axis*

Writing an equation for a circle: To write an equation for a circle, it first needs to be put on a graph. Let's make it easy and have the centre of the circle at the origin of the graph (0,0).

The equation of a circle looks like this: [1/r (x-h)]^2 + [1/r (y-k)]^2 = 1  This is called Transformational Form. The r stands for radius, h is the horizontal translation and k is the vertical translation. h and k is also the centre of the circle.

The equation for the circle above circle is: [1/5 (x-0)]^2 + [1/5 (y-0)]^2 = 1

If we only had the equation and not the diagram of the circle, we would need a mapping rule to make the diagram. A mapping rule looks like this:

(x,y) -> (x+h, ay+k) Notice how the h and k are now positive? This becomes important later. The a stands for the stretch, but it not needed here.

The mapping rule for the above circle is: (x,y) -> (x+0, y+0)

Let's make it harder and change the circle on the graph:

The equation fo this circle is: [1/5 (x-0)]^2 + [1/5 (y-3)]^2 =1  All we did with this circle was move it vertically by +3. This +3 is equal to k. This is what happened: [1/5 (x-(0)]^2 + [1/5 (y-(+3)]^2 =1 We did not move the circle horizontally from our last circle so it stays at 0.

The mapping rule now is: (x,y) -> (x+0, y+3) In the mapping rule, the h and k are positive, unlike the equation where they are negative. This is to show that the circle moved +3 on the y-axis. It makes it easier to draw.

Writing an equation for an ellipse: The equation for an ellipse is the same as an equation for a circle except we now have to use a stretch. We'll start off with an ellipse with a centre at the origin (0,0).

The equation of an ellipse is: [1/a (x-h)]^2 + [1/b (y-k)]^2 =1 This is also called transformational form. In this equation, the only difference from the circle equation is the a and b. Both a and b stand for stretch. a is usually the longer stretch but it does not matter.

The equation for the above ellipse is: [1/3 (x-0)]^2 + [1/6 (y-0)]^2 =1

The mapping rule for the above ellipse is: (x,y) -> (x+0, 2y+0) Because this ellipse has a longer y-axis, the stretch goes with the y in the mapping rule. In this ellipse, the y-axis is twice the length of the x-axis, giving it a stretch of 2.