Let AB be the parabolic arch having O at the vertex and the vertical line OY as the axis.
The parabola open upwards
∴  Its equation is of the form  ...(i)
   Width of the arch, LM = 5 m
              OM = 2.5 m
   Height of the arch, BM = 10 m
∴ Co-ordinates of point B are (2.5, 10)
Since point B lies on the parabolaÂ
∴   Â
∴   From (i), the equation of the parabola is:Â
or                                           ...(ii)
We have to find the width PQ of the arch at a distance ON = 2 m from the vertex.
Let            PQ = d  NQ =Â
∴   Co-ordinates of point Q areÂ
Putting it in (ii), we getÂ
Hence, the width of the arch = d =Â Â or 2.23 (approx).
                          Â
If a parabolic reflector is  in diameter and 20 cm deep, find the distance of its focus S from the vertex.
An equilateral triangle is inscribed in a parabola y2Â = 16x, where one vertex is at the vertex of the parabola. Find
(i) Length of each side of the triangle (ii) the area of the triangle