Mathematics

Question

Using a directrix of y = 5 with focus at (4, 1), what quadratic function is created?

f(x) = 1/4(x − 4)2 − 3
f(x) = 1/8(x + 4)2 − 3
f(x) = −1/8(x − 4)2 + 3
f(x) = -1/4(x + 4)2 − 3

1 Answer

  • Answer:

    C

    Step-by-step explanation:

    From any point (x, y) on the parabola the focus and directrix are equidistant.

    Using the distance formula

    [tex]\sqrt{(x-4)^2+(y-1)^2}[/tex] = | y - 5 |

    Squaring both sides

    (x - 4)² + (y - 1)² = (y - 5)² ← distribute the factors in y

    (x - 4)² + y² - 2y + 1 = y² - 10y + 25 ( subtract y² - 10y + 25 from both sides )

    (x - 4)² + 8y - 24 = 0  ( subtract (x - 4)² from both sides )

    8y - 24 = - (x - 4)² ← add 24 to both sides )

    8y = - (x - 4)² + 24 ( divide both sides by 8 )

    y = - [tex]\frac{1}{8}[/tex] (x - 4)² + 3

    Hence

    f(x) = - [tex]\frac{1}{8}[/tex] (x - 4)² + 3 → C

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