Using a directrix of y = -2 and a focus of (2, 6), what quadratic function is created?
f(x) = -1/8(x - 2)2 - 2
f(x) = 1/16(x - 2)2 + 2
f(x) = 1/8(x - 2)2 - 2
f(x) = -1/16(x + 2)2 - 2
Solution:
The definition of a parabola states that all points on the parabola always have the same distance to the focus and the directrix.
Given: Focus, F = (2, 6) and directrix = -2
Let A = (x, y) be a point on the parabola.
D = (x, -2) represent the closest point on the directrix
First, find out the distance using distance formula,
d = √(x2 - x1)2 + (y2 - y1)2
Distance A and F is dAF = √(x - 2)2 + (y - 6)2
Distance between A and D is dAD = √(x - x)2 + (y - (-2))2 = √(y + 2)2
Since these distances must be equal to each other,
√(x - 2)2 + (y - 6)2 = √(y + 2)2
Squaring both sides,
⇒ (√(x - 2)2 + (y - 6)2)2 = (√(y + 2)2)2
⇒ (x - 2)2 + (y - 6)2 = (y + 2)2
⇒ (x - 2)2 + y2 - 12y + 36 = y2 + 4y + 4.
⇒ (x - 2)2 + 36 - 4 - 12y - 4y = 0
⇒ (x - 2)2 + 32 - 16y = 0
⇒ 16y = (x - 2)2 + 32
⇒ y = 1/16 [(x - 2)2 + 2]
Therefore, the quadratic function is y = 1/16 [(x - 2)2 + 2].
Using a directrix of y = -2 and a focus of (2, 6), what quadratic function is created?
Summary:
Using a directrix of y = -2 and a focus of (2, 6), the quadratic function created is f(x) = 1/16 [(x - 2)2 + 2].
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