Posted by victor on .
Find the equation of the parabola with the vertex at the origin and directrix y =5

mathematics 
Henry,
The vertex is midway between the directrix and focal point:
D(0,5)
V(0,0)
F(0,y)
FV = VD = 1/4a = 5,
0  y = 5,
Y = 5.
1/4a = 5,
Multiply both sides by 4a:
20a = 1,
a = 1/20.
Vertex Form: Y = a(xh)^2 + k,
Y = 1/20(x0)^2 + 0,
Eq: Y =(1/20)x^2