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In the previous example, we noticed that the x- and y-intercepts of the graph of an ellipse centred at the origin could be determined from its equation. Another way of examining the equation and its graph is by examining the horizontal and vertical axes of the ellipse.
Recall that an ellipse has two axes of symmetry.
We will call the axis of symmetry parallel to the x-axis the horizontal axis. The axis of symmetry parallel to the y-axis will be called the vertical axis.
The vertical and horizontal axes intersect at the centre of the ellipse.

Example 1 from Page 2 4x2 + 25y2 = 100	Looking at the graph of 4x2 + 25y2 = 100 from the previous page, we see that the length of the horizontal axis is 10 and the length of the vertical axis is 4.
	How are these lengths related to the equation x2/ 25 + y2 / 4 = 1?
	Notice that the length of the horizontal axis is 10 = 2(Ö25) and the length of the vertical axis is 4 = 2(Ö4).
Example 2 from Page 2 16x2 + 9y2 = 225	In this ellipse the length of the horizontal axis is 6 and the length of the vertical axis is 8.
	Again, notice the relationship between these lengths and the values in the denominators in the equation x2/ 9 + y2/ 16 = 1.
	The length of the horizontal axis is 6 = 2(Ö9) and the length of the vertical axis is 8 = 2(Ö16).

How does varying the values of a and b in the equation x2/ a2 + y2/ b2 = 1 effect the shape of the graph?