Geometry · real student question

Identify the vertex, focus, axis of symmetry, and directrix for the parabola (x - 1)^2 = 8(y - 2).

Question

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

(x1)2=8(y2)(x-1)^2 = 8(y-2)

Step-by-step solution

  1. Decide which of the two standard forms applies. The squared variable is xx, so the parabola opens vertically. That fixes the template as

    (xh)2=4p(yk)(x-h)^2 = 4p(y-k)

    If yy were the squared variable the parabola would open sideways and every formula below would swap roles.

  2. Read off hh, kk and pp by matching term for term. Comparing (x1)2=8(y2)(x-1)^2 = 8(y-2) with the template gives h=1h = 1, k=2k = 2 and 4p=84p = 8, so

    p=2p = 2

    Since p>0p > 0 the parabola opens upward.

  3. The vertex is (h,k)(h,k). Straight from the match, the vertex is (1,2)(1, 2).

  4. The focus sits pp units from the vertex along the axis. For a vertical parabola the focus is (h,  k+p)(h,\; k+p):

    (1,  2+2)=(1,4)(1,\; 2+2) = (1, 4)

  5. The directrix sits pp units on the other side, and the axis passes through both. The axis of symmetry is the vertical line through the vertex, x=h=1x = h = 1. The directrix is the horizontal line y=kp=22=0y = k - p = 2 - 2 = 0, i.e. the xx-axis.

  6. Verify with the focus–directrix definition. Take (5,4)(5, 4): (51)2=16=8(42)(5-1)^2 = 16 = 8(4-2), so the point really is on the parabola. Its distance to the focus (1,4)(1,4) is 44, and its distance to the directrix y=0y = 0 is also 44. Equal distances confirm both the focus and the directrix.

Answer

Vertex (1,2),Focus (1,4),Axis x=1,Directrix y=0\text{Vertex } (1,2),\quad \text{Focus } (1,4),\quad \text{Axis } x=1,\quad \text{Directrix } y=0

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