Question

How do I know whether the major axis of an ellipse is horizontal or vertical?

Answer 1

Locate the larger of the 2 denominators. Then look at that numerator.

If it has an `x` then that is the major axis. Horizontal
If it has an `y` then that is the major axis. Vertical

`x^2/4+y^2/16=1`

The major axis is `y`. Vertical

`x^2/25+y^2/16=1`

The major axis is `x`. Horizontal