Finding Equation of Smallest Circle Containing 3 Circles

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Homework Help Overview

The problem involves finding the equation of the smallest circle that can contain three given circles, each defined by their equations. The context is centered around geometric relationships and properties of circles.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the equations of the circles and explore the concept of finding a circumcenter for the triangle formed by the centers of the circles. There is a request for hints and clarification on how to approach the problem with three circles, as well as considerations of special relationships between the circles.

Discussion Status

Some participants have offered insights regarding the circumcenter and its relevance to the problem, while others have pointed out the difficulty of the problem without specific relationships among the circles. Multiple interpretations of the problem are being explored, and hints have been provided without reaching a consensus.

Contextual Notes

The equations of the circles have been provided, along with their centers and radii. There is an acknowledgment that the problem may be more manageable if the circles have certain geometric properties.

phymatter
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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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