Find the directrix for the following: -12(y - 2) = (x + 1)2

Question

Find the directrix for the following:

-12(y - 2) = (x + 1)2

Answer 1

Moon,

 

The equation is already in standard form:

 

4p(y − k) = (x − h)², 

 

with h = −1 and k = 2.

 

Since 4p = −12, p = −3. The vertex is (−1, 2) and the parabola opens downward (since the x-term is squared and p < 0), so the directrix is above the vertex. Go 3 units above the vertex and find the horizontal line. It is y = 5.