# Circle on Plane

• Apr 22nd 2007, 09:31 AM
r_maths
Circle on Plane
Hey, me again.

The centres A,B, and C of the three circles are collinear. (A is smallest circle, B is biggest, C is 2nd biggest)

http://img441.imageshack.us/img441/5105/circlelp9.png

The equation of circle A is: (x + 12)² + (y + 15)² = 25
The equation of circle C is: (x - 24)² + (y - 12)² = 100

Find the equation of circle B.

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I done the easy part of finding the radius of Circle B (15). The bit i'm stuck at is finding the centre of circle B, which from the answer is (4,-3) . Thanks

The answer is: x² + y² - 8x + 4y + 200 = 0
• Apr 22nd 2007, 09:50 AM
Plato
If (h,k) is the center of B then the distance from (-12,-15) to (h,k) is 20.
The slope (k+15)/(h+12) must be 3/4.
• Apr 22nd 2007, 09:58 AM
r_maths
Quote:

Originally Posted by Plato
If (h,k) is the center of B then the distance from (-12,-15) to (h,k) is 20.
The slope (k+15)/(h+12) must be 3/4.

I don't quite understand the part in bold, unless your talking about the equation of the line.
A(-12, -15) C(24, 12) m = 3/4
4y = 3x - 24

Could you say what the next step is, in finding the centre of B.
• Apr 22nd 2007, 10:28 AM
Jhevon
Quote:

Originally Posted by r_maths
Hey, me again.

The centres A,B, and C of the three circles are collinear. (A is smallest circle, B is biggest, C is 2nd biggest)

http://img441.imageshack.us/img441/5105/circlelp9.png

The equation of circle A is: (x + 12)² + (y + 15)² = 25
The equation of circle C is: (x - 24)² + (y - 12)² = 100

Find the equation of circle B.

------

I done the easy part of finding the radius of Circle B (15). The bit i'm stuck at is finding the centre of circle B, which from the answer is (4,-3) . Thanks

The answer is: x² + y² - 8x + 4y + 200 = 0

There are actually many ways to do this problem, this is the easiest way i can see at the moment, but then i never thought about it very long.

The radius of A is 5, the radius of C is 10 (do you see that?)
the center of A is (-12,-15), the center of C is (24,12) (do you see that?)

what is the distance between the two centers?

d = sqrt[(24 + 12)^2 + (12 + 15)^2] = sqrt(1296 + 729) = 45

since the radius of A is 5 and the radius of C is 10, then the diameter of B must be:

45 - 5 - 10 = 30
so the radius of B is 15.

now the slope of the line connecting the center of A and the center of B is:
m = (y2 - y1)/(x2 - x1) = (12 + 15)/(24 + 12) = 27/36 = 3/4

so notice that if we move 4 units horizontally from the center of A and 3 units up as the slope suggests, we end up with a 3-4-5 triangle. the hypotenuse of this triangle will be 5, which is the exact radius of A. the distance between the center of A and the center of B is 20. so the hypotenuse of the triangle that connects the center of A to the center of B is 20. that is we multiply 5 by 4. since this will be a similar triangle to the first, we can multiply the other 2 numbers by 4. that is we move 4*4 units across and 3*4 units up. that is 16 units across and 12 units up. so the center of B will be given by:
where (x,y) is the center of A

(or we could use: 20/y = 5/3 and 20/x = 5/4 to find the x and y units we move from the center of A)

(x + 16, y + 12) = (-12 + 16,-15 + 12) = (4,-3).

Now we know the radius of B and it's center.

so the equation of B is:

(x - 4)^2 + (y + 3)^2 = 15^2
=> x^2 - 8x + 16 + y^2 + 6y + 9 = 225
=> x^2 - 8x + y^2 + 6y - 200 = 0

another way: we can find the center of B by finding the point (x,y) that is 20 units from the center of A and at the same time is 25 units from the center of C

so we have:

25 = sqrt[(x - 24)^2 + (y - 12)^2] ..................(1)
20 = sqrt[(x + 12 )^2 + (y + 15 )^2] ...............(2)

solving this system should give you the center coordinates, but that was just too much work

also, you could find the equation of the line:

what is the line connecting these three points?
let's find the slope first:
m = (y2 - y1)/(x2 - x1) = (12 + 15)/(24 + 12) = 27/36 = 3/4

using the point slope form with m = 3/4 and (x1,y1) = (24,12), we get:
y - y1 = m(x - x1)
=> y - 12 = (3/4)(x - 24)
=> y = (3/4)x - 18 + 12
=> y = (3/4)x - 6

and use the distance formula with the relevant data to get the points for the center of B, using the equation of the line as a constraint, again, too much work. simialr triangles seems easiest.

i can attach a diagram if you want to see what similar triangles i am talking about.

did you get my solution?
• Apr 22nd 2007, 10:33 AM
Plato
Quote:

Originally Posted by r_maths
about the equation of the line.
A(-12, -15) C(24, 12) m = 3/4
4y = 3x - 24

(h+12)^2 + (y+15)^2 = 20^2 and 4(k+15)=3(h+12).
Now solve for (h,k).
• Apr 22nd 2007, 12:03 PM
r_maths
Quote:

Originally Posted by Jhevon
did you get my solution?

Yep. I know what sim. triangle your talking about, Thanks for your time.