# Thread: finding equation of a circle and parabola

1. ## finding equation of a circle and parabola

Finding the general and standard form equations for the circle and parabola passing through thee points (4,6), (-6,2), and (1,-3).

I used the y=Ax^2+Bx+C to find a,b,c.

(4,6)=> 6=A(4)^2+B(4)+C
(-6,2)=> 2=A(-6)^2+B(-6)+C
(1,-3)=> -3A(1)^2+B(1)+C

then,
a=13/15, b=8/7, and c=-158/35

so the equation for parabola:
$y=(13/35)x^2+(8/7)x-(158/35)$

how do I make this equation into this equation f(x)a(x-h)^2+k without the vertex?

2. Originally Posted by Anemori
Finding the general and standard form equations for the circle and parabola passing through thee points (4,6), (-6,2), and (1,-3).

I used the y=Ax^2+Bx+C to find a,b,c.

(4,6)=> 6=A(4)^2+B(4)+C
(-6,2)=> 2=A(-6)^2+B(-6)+C
(1,-3)=> -3A(1)^2+B(1)+C

then,
a=13/15, b=8/7, and c=-158/35

so the equation for parabola:
$y=(13/35)x^2+(8/7)x-(158/35)$

how do I make this equation into this equation f(x)a(x-h)^2+k without the vertex?
x-value of the vertex ... $h = \frac{-b}{2a}$

... and $k = f(h)$

3. Originally Posted by skeeter
x-value of the vertex ... $h = \frac{-b}{2a}$

... and $k = f(h)$

I see, thanks!

Should I use the same a,b,c value from parabola for finding the equation for a circle?

4. Originally Posted by Anemori
Should I use the same a,b,c value from parabola for finding the equation for a circle?
the equation for a circle is $(x-h)^2 + (y-k)^2 = r^2
$
... where will that idea get you?

I would find the circle's center (h,k) by locating the intersection of the perpendicular bisectors of at least two chords formed by the three points.

5. Originally Posted by skeeter
the equation for a circle is $(x-h)^2 + (y-k)^2 = r^2
$
... where will that idea get you?

I would find the circle's center (h,k) by locating the intersection of the perpendicular bisectors of at least two chords formed by the three points.

I tried to find the equation for the circle and parabola from this three points (4,6),(-6,2), and (1,-3). this is what i get:

for parabola:
general form=
$y=\frac{13}{35}x^2+\frac{8}{7}x-\frac{158}{35}$
standard form=
$y=\frac{13x^2-158}{35}+\frac{8x}{7}$

Vertex: (158,8x/7)
focus: (58, 8x/7, -35/4)
directrix: y=8x/7 -35/4

for circle:

$(x-\frac{77}{6})^2+(y-\frac{85}{22})^2 = 13.4^2$

center= (77/6, 85/22)

6. Originally Posted by Anemori
I tried to find the equation for the circle and parabola from this three points (4,6),(-6,2), and (1,-3). this is what i get:

for circle:

$(x-\frac{77}{6})^2+(y-\frac{85}{22})^2 = 13.4^2$

center= (77/6, 85/22)

sub in the values x = 4 y = 6 into the circle equation ... does it work?

7. Originally Posted by skeeter
sub in the values x = 4 y = 6 into the circle equation ... does it work?
nope >< man i suck!

8. The general equation of the circle passing through (4, 6), (-6, 2) and (1, -3) is

$x^2 + y^2 +2gx + 2fy + c = 0$

Substitute the points in the above equation, and solve for g, f and c. I got the equation of the circle as

$13x^2 + 13y^2 + 10x -32y - 36 = 0$

Check it.
Consider any one point as the common point for circle and parabola.
Let it be the vertex. Find the equation of the tangent to the circle at the vertex in the form ax +by + c = 0. Diretrix of the parabola will be parallel to this tangent in the form ax + by + c' = 0.
Let (h, k) be the focus. In the parabola, the distance between any point and the focus is equal the distance between the point and diretrix.

$(h-4)^2 + (k-6)^2 = \frac{(4a + 6b + c')^2}{(a^2 + b^2)}$

Write down three equations for three points and solve for h, k and c'

9. Now we need to find the circle: (x - h)^2 + (y - k)^2 = r^2
center = (h,k)

So we solve another system of 3 equations:
(4 - h)^2 + (6 - k)^2 = r^2
(-6 - h)^2 + (2 - k)^2 = r^2
(1 - h)^2 + (-3 - k)^2 = r^2

[(4 - h)^2 + (6 - k)^2 = r^2] - [(-6 - h)^2 + (2 - k)^2 = r^2] = (-20h - 8k + 12 = 0)
[(4 - h)^2 + (6 - k)^2 = r^2] - [(1 - h)^2 + (-3 - k)^2 = r^2] = (-6h - 18k + 42 = 0)

Now solve the system of two equations:
(-20h - 8k + 12 = 0)
(-6h - 18k + 42 = 0)

h = (-5/13)
k = (32/13)

r = sqrt(5365)/13

x + (5/13)]^2 + [y - (32/13)]^2 = [sqrt(5365)/13]^2

this is what i got.......