If an elliptic race track is 100 yards long and 20 yards wide, then how far apart are the foci?

Question

a. 

b. 

c. 

d. 

Answer 1

For an ellipse: a²-b² = c² where a is the semi-major axis, b is the semi-minor axis and c is the focal length.

The foci are 2c apart, so the answer is

2sqrt( 50²-10²) which you can match with one of the answers.