An ellipse has 2 focus points. The sum of the distance from each of these two points to any point on the ellipse is constant.
The standard form is: x2/a2 + y2/b2 = 1
(For major axes along the x and y axes.)
Where a is the distance from the center of the ellipse to the outer edge along the x axis and b is the the distance from the center of the ellipse to the outer edge along the y axis.
EXAMPLE: Sketch 4x2 + 9y2 = 36
First we need to rearrange the equation into a form that looks like the standard form. How?
Divide the whole equation by 36.
x2/9 + y2/4 = 1