What is the directrix y=-x^2+18x-79 ?

The directrix y=-x^2+18x-79 is y= 9/4

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directrix y=-x^2+18x-79

directrix

y = - x^{2} + 18 x - 79

y = \frac{9}{4}

Solution steps

Rewrite

y = - x^{2} + 18 x - 79

in the standard form

4 (- \frac{1}{4}) (y - 2) = (x - 9)^{2}

(h, k) = (9, 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

(9, 2)

y-coordinate

y = 2 - p

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

Refine

y = \frac{9}{4}

y = (x - 2)^{2} - 7

y > x^{2} - 3

x^{2} + 20 y = 10

2 y^{2} = - 3 x

x^{2} + 8 x + 7