Question 200171
y =  (-1/2) x^2
.
-2y = x^2,,,,,but  x^2 = 4py  (  standard  form)
.
4py = -2y
.
p = -1/2 ,,,,,,focus (0,p),,,, or  focus ( 0 , -1/2)
.
Directrix = -p  (standard  form),,,,Directrix  = - (-1/2),,,or  directrix  =  1/2
.