Finding Equation of Smallest Circle Containing 3 Circles

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SUMMARY

The discussion focuses on finding the equation of the smallest circle that contains three given circles defined by their equations. The circles provided are: 1. x² + y² - 4y - 5 = 0, 2. x² + y² + 12x + 4y + 31 = 0, and 3. x² + y² + 6x + 12y + 36 = 0. The centers of these circles are located at (0,2), (-6,-2), and (-3,-6) with a radius of 3 for each. The solution involves calculating the circumcenter of the triangle formed by the centers of the circles, which will be equidistant from all three vertices, and then determining the radius of the containing circle as the distance from the circumcenter to any center plus the radius of the circles.

PREREQUISITES
  • Understanding of circle equations in Cartesian coordinates
  • Knowledge of circumcenters and centroids in geometry
  • Familiarity with the Pythagorean theorem
  • Ability to solve quadratic equations
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  • Study the properties of circumcenters and centroids in triangles
  • Learn how to derive the equations of circles from their standard forms
  • Explore methods for calculating distances between points in a Cartesian plane
  • Investigate the relationship between the circumradius and the triangle formed by circle centers
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Students studying geometry, mathematicians focusing on circle properties, and anyone tackling problems involving multiple circles in coordinate systems.

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Homework Statement



if equation of 3 circles is given then find the equation of the smallest circle containing all 3 circles

Homework Equations





The Attempt at a Solution



i can do this question for 2 circles , please give a hint for 3 circles
 
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What are the equations? There is an easy way to tell which is the 'smallest' circle given the equations. Hint: Look at the constants, especially the one after the minus sign in the radical.
 
Angry Citizen said:
What are the equations? There is an easy way to tell which is the 'smallest' circle given the equations. Hint: Look at the constants, especially the one after the minus sign in the radical.
I think you misunderstood the problem.

phymatter said:

Homework Statement



if equation of 3 circles is given then find the equation of the smallest circle containing all 3 circles

Homework Equations





The Attempt at a Solution



i can do this question for 2 circles , please give a hint for 3 circles
The problem is very difficult to solve if the circles don't have some special relationship such as they are all touching at one point each for example.
 
the given equations are :
1. x2 +y2 -4y-5 = 0
2. x2 +y2 + 12x +4y +31 = 0
3. x2 +y2 +6x +12y +36 = 0

the centres are (0,2) ; (-6,-2) ; (-3,-6) and radius of all is 3
 
I haven't worked this through, but I believe what you need is the circumcenter of the triangle formed from the centers of the three circles. That point would be equidistant from the three vertices of the triangles, so that would be the center of the circle that surrounds the three given circles. After you know the coordinates of the circumcenter, you can find the distance, call it d, to any of the centers of the three given circles. The radius of your containing circle will be d + 3.
 
The triangle is an equilateral triangle...circumcentre is same as centroid...
Remember the centroid ratio and pythagoras theorem...
I hope this will help you!

Thanx...
Suk-Sci
 

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Last edited:
Suk-Sci said:
The triangle is an equilateral triangle...circumcentre is same as centroid...
Remember the centroid ratio and pythagoras theorem...
I hope this will help you!

Thanx...
Suk-Sci

thanks Suk-Sci !
 

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