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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=12−12xy^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=12−12x(y+3)^2 = 12 - 12x

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

    (y+3)2=−12(x−1)(y+3)^2 = -12(x-1)

    The sign matters — writing 12(1−x)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 (y−k)2=4p(x−h)(y-k)^2 = 4p(x-h). Writing (y−(−3))2=−12(x−1)(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=h−p=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(x−1)(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 ∣−2−4∣=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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