What is the directrix y=-x^2+8x-18 ?

The directrix y=-x^2+8x-18 is y=-7/4

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directrix y=-x^2+8x-18

directrix

y = - x^{2} + 8 x - 18

y = - \frac{7}{4}

Solution steps

Rewrite

y = - x^{2} + 8 x - 18

in the standard form

4 (- \frac{1}{4}) (y - (- 2)) = (x - 4)^{2}

(h, k) = (4, - 2), p = - \frac{1}{4}

Parabola is symmetric around the y-axis and so the directrix is a line parallel to the x-axis, a distance

- p

from the center

(4, - 2)

y-coordinate

y = - 2 - p

y = - 2 - (- \frac{1}{4})

Refine

y = - \frac{7}{4}