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Geometrically speaking, a parabola is defined as the set of points that are the same distance from a given point and a given line. The point is called the focus of the parabola and the line is called the directrix of the parabola.

 

Suppose P is a parabola with focus (4,3) and directrix y=1. The point (8,6) is on P because (8,6) is 5 units away from both the focus and the directrix. If we write the equation whose graph is P in the form y=ax^2 + bx + c, then what is a+b+c?

 Jul 20, 2020
 #1
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I've answered the first one elsewhere today........

 

Geometrically speaking, a parabola is defined as the set of points that are the same distance from a given point and a given line. The point is called the focus of the parabola and the line is called the directrix of the parabola. Suppose P is a parabola with focus (4,3) and directrix y=1 . The point (8,6) is on P because (8,6)  is 5 units away from both the focus and the directrix. If we write the equation whose graph is P in the form y=ax^2 + bx + c, then what is a*b*c ?

 

The vertex  will be found at (4,2)   and we can write that :

 

(y - 2) = (a)(x - 4)^2       and since (8,6)   is on the curve, we can solve for a

 

(6 -2)  = (a) (8 - 4)^2    simplify

 

4 = (a)(4)^2

 

4 = (a)16   →    a = 4/16  = 1/4

 

Since the x coordinate of the vertex is given by -b/ (2a)  we have that   -b/[2 (1/4)]  = 4   → -b / (1/2) = 4 → -b = 2 → b = -2

 

And using the fact that  (4,2)  is on the graph, we can find c, thusly :

 

y = ax^2 + bx + c  ......so......

 

2 = (1/4)(4)^2 -2(4) + c

 

2 = 4 - 8 + c

 

2  = -4 + c

 

c = 6

 

Then a*b*c =  (1/4) (-2) (6)    =  (1/4)(-12)    = -3

 Jul 20, 2020
 #3
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Good work....but   a + b + c

ElectricPavlov  Jul 20, 2020
 #2
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+1

You are given the directrix and the focus      you should be able to deduce that the vertex (between the two)

   is at    4,2   this is h,k

 

Parabola in vetex form      y = a(x-h)^2 + k

y = a(x-4)^2 + 2   Sub in th epoint given to calc 'a'

6 = a(8-4)^2 + 2

a = 1/4

 

now your equation becomes    y = 1/4 (x-4)^2 + 2    expand

                                                    = 1/4 x^2 -2x+6      now you can see what a b c are  to add them.....

 Jul 20, 2020
 #4
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I did 1/4 + -2 + 6 and got 4 1/4, but the answer is not a fraction. Can you check your work? Thanks!

 Jul 20, 2020
 #5
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-1

Nevermind Thanks I got it

Guest Jul 20, 2020

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