x² + 4x - 8y + 36 = 0
Since the x is the variable that is squared, get all terms
in x on the left and other terms on the right:
x² + 4x = 8y - 36
Complete the square on the left
1. Multiply the coefficient of x, which is 4, by
, getting 2
2. Square that result, getting 4
3. Add to both sides.
x² + 4x + 4 = 8y - 36 + 4
Factor the left side, collect like terms on the right
(x + 2)(x + 2) = 8y - 32
Write the left side as a perfect square. Factor 8 out on the right
(x + 2)² = 8(y - 4)
Compare to
(x - h)² = 4p(y - k)
with vertex (h,k) = (-2,4) 4p = 8, so p = 2
We plot the vertex
Since p is positive, the parabola opens upward. the directrix
is p=2 units below the vertex and has the equation y = k-p or
y = 4-2 or y=2
It is the green line below
The focus is p=2 units above the vertex (h,k+p) or (-2,4+2) or (2,6)
Draw a line from the focus directly to the directrix and draw 2
squares on each side of that line, like this.
Then sketch in the parabola through the vertex and the upper outer
corners of the two squares:
Edwin