Explanation

The foci are two points inside the ellipse that help define its shape.

Formula:
c² = a² − b²

Where:

• c = distance from center to each focus

If the major axis is horizontal:
Foci: (±c, 0)

If vertical:
Foci: (0, ±c)

Key property:
The sum of distances from any point on the ellipse to both foci is constant.

Quiz

1. How many foci does an ellipse have?

2. What formula finds c?

3. Are the foci inside or outside the ellipse?

4. Where are the foci if the major axis is vertical?

5. What stays constant for an ellipse?

Answer Key

1. Two

2. c² = a² − b²

3. Inside

4. (0, ±c)

5. Sum of distances to the foci