Geometry · real student question

Identify the vertex, focus, axis of symmetry, and directrix for the parabola y^2 + 6y + 9 = 12 - 12x.

Question

Identify the vertex, focus, axis of symmetry, and directrix for the parabola

y2+6y+9=1212xy^2 + 6y + 9 = 12 - 12x

Step-by-step solution

  1. Spot the perfect-square trinomial instead of completing the square. The left side y2+6y+9y^2 + 6y + 9 is already (y+3)2(y+3)^2, because 9=(6/2)29 = (6/2)^2. Recognising this skips the completing-the-square step entirely:

    (y+3)2=1212x(y+3)^2 = 12 - 12x

  2. Factor the right side so it looks like 4p(xh)4p(x-h). Pull out 12-12:

    (y+3)2=12(x1)(y+3)^2 = -12(x-1)

    The sign matters — writing 12(1x)12(1-x) instead would hide the value of hh.

  3. Match to the horizontal template. Here yy is the squared variable, so the parabola opens sideways and the form is (yk)2=4p(xh)(y-k)^2 = 4p(x-h). Writing (y(3))2=12(x1)(y-(-3))^2 = -12(x-1) gives h=1h = 1, k=3k = -3 and 4p=124p = -12, so

    p=3p = -3

    Because p<0p < 0 the parabola opens left.

  4. Locate the vertex and focus. The vertex is (h,k)=(1,3)(h,k) = (1,-3). For a horizontal parabola the focus is (h+p,  k)(h+p,\; k):

    (1+(3),3)=(2,3)(1 + (-3),\, -3) = (-2, -3)

  5. Give the axis and directrix. The axis of symmetry is the horizontal line through the vertex, y=k=3y = k = -3. The directrix is the vertical line x=hp=1(3)=4x = h - p = 1 - (-3) = 4, three units to the right of the vertex — on the opposite side from the focus, as it must be.

  6. Check one point against the definition. At y=3y = 3 the equation gives (3+3)2=36=12(x1)(3+3)^2 = 36 = -12(x-1), so x=2x = -2 and (2,3)(-2, 3) lies on the curve. Its distance to the focus (2,3)(-2,-3) is 66, and its distance to the directrix x=4x = 4 is 24=6|-2 - 4| = 6. The two distances agree.

Answer

Vertex (1,3),Focus (2,3),Axis y=3,Directrix x=4\text{Vertex } (1,-3),\quad \text{Focus } (-2,-3),\quad \text{Axis } y=-3,\quad \text{Directrix } x=4

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