9514 1404 393
Answer:
11. y = -1/4x² +1
12. y = 1/6(x -3)² +5/2
Step-by-step explanation:
Since 9-12 are all the same, we can solve the problem generically and plug in the given numbers. Let the focus be (a, b) and the directrix be y=d.
The vertex is halfway between the focus and directrix, so its coordinates will be (a, (b+d)/2). The distance from focus to directrix is b-d. Given these conditions, the equation of the parabola is ...
y = 1/(2(b-d))(x -a)² +(b+d)/2
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11. For (a, b) = (0, 0) and d = 2, the equation is ...
y = 1/(2(0-2))(x -0)² +(0+2)/2
y = -1/4x² +1
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12. For (a, b) = (3, 4) and d = 1, the equation is ...
y = 1/(2(4-1))(x -3)² +(4+1)/2
y = 1/6(x -3)² +5/2
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The attachments show the graphs of these. You can verify the graph by noticing ...
a) the vertex is halfway between the focus and directrix
b) a horizontal line from the focus to the curve is the same length as a vertical line from the directrix to the same point on the curve
