Question
The arch given below is in form of a semi-ellipse. It is 8 m wide and its height at the centre is 2 m. Find the height of the arch at a distance of 1.5 m from one end.

Solution
Width of the elliptical arch = 8 m AA' = 8m
2a = 8m
a = 4
Height at the centre = 2m OB = 2m
b = 2
∴ the equation of ellipse is ...(i)
Let the height at P be h.
∴ Coordinates of L are (2.5, h)
It lies on ellipse (i)
Hence,