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Length of the perpendicular bisector of the line joining the centers of two circles
  • Last Updated : 22 Mar, 2021

Given are two circles whose radii are given, such that the smaller lies completely within the bigger circle, and they touch each other at one point. We have to find the length of the perpendicular bisector of the line joining the centres of the circles.
Examples: 
 

Input: r1 = 5, r2 = 3
Output: 9.79796

Input: r1 = 8, r2 = 4
Output: 15.4919

 

Approach
 

  • Let the two circles have center at A and B.The perpendicular bisector PQ, bisects the line at C
     
  • Let radius of bigger circle = r1 
    radius of smaller circle = r2 
     
  • so, AB = r1-r2
     
  • therefore, AC = (r1-r2)/2 
     
  • In, the figure, we see 
    PA = r1 
     
  • in triangle ACP
    PC^2 + AC^2 = PA^2 
    PC^2 = PA^2 – AC^2 
    PC^2 = r1^2 – (r1-r2)^2/4 
     
  • so, PQ = 2*√(r1^2 – (r1-r2)^2/4) 
     

 



Length of the perpendicular bisector = 2 * sqrt(r1^2 – (r1-r2)*(r1-r2)/4)

Below is the implementation of the above approach: 
 

C++




// C++ program to find the Length
// of the perpendicular bisector
// of the line joining the centers
// of two circles in which one lies
// completely inside touching the
// bigger circle at one point
 
#include <bits/stdc++.h>
using namespace std;
 
void lengperpbisect(double r1, double r2)
{
    double z = 2 * sqrt((r1 * r1)
                        - ((r1 - r2)
                           * (r1 - r2) / 4));
 
    cout << "The length of the "
         << "perpendicular bisector is "
         << z << endl;
}
 
// Driver code
int main()
{
    double r1 = 5, r2 = 3;
    lengperpbisect(r1, r2);
    return 0;
}

Java




// Java program to find the Length
// of the perpendicular bisector
// of the line joining the centers
// of two circles in which one lies
// completely inside touching the
// bigger circle at one point
 
class GFG {
     
static void lengperpbisect(double r1, double r2)
{
    double z = 2 * Math.sqrt((r1 * r1)
                        - ((r1 - r2)
                        * (r1 - r2) / 4));
 
    System.out.println("The length of the "
        + "perpendicular bisector is "
        + z );
}
 
// Driver code
public static void main(String[] args)
{
    double r1 = 5, r2 = 3;
    lengperpbisect(r1, r2);
}
}
 
// This code has been contributed by 29AjayKumar

Python3




     
# Python program to find the Length
# of the perpendicular bisector
# of the line joining the centers
# of two circles in which one lies
# completely inside touching the
# bigger circle at one point
 
def lengperpbisect(r1, r2):
    z = 2 * (((r1 * r1) - ((r1 - r2) * (r1 - r2) / 4))**(1/2));
 
    print("The length of the perpendicular bisector is ", z);
 
 
# Driver code
r1 = 5; r2 = 3;
lengperpbisect(r1, r2);
 
# This code contributed by PrinciRaj1992

C#




// C# program to find the Length
// of the perpendicular bisector
// of the line joining the centers
// of two circles in which one lies
// completely inside touching the
// bigger circle at one point
using System;
 
class GFG
{
     
static void lengperpbisect(double r1, double r2)
{
    double z = 2 * Math.Sqrt((r1 * r1)
                        - ((r1 - r2)
                        * (r1 - r2) / 4));
 
    Console.WriteLine("The length of the "
        + "perpendicular bisector is "
        + z );
}
 
// Driver code
public static void Main()
{
    double r1 = 5, r2 = 3;
    lengperpbisect(r1, r2);
}
}
 
// This code has been contributed by anuj_67..

Javascript




<script>
// javascript program to find the Length
// of the perpendicular bisector
// of the line joining the centers
// of two circles in which one lies
// completely inside touching the
// bigger circle at one point
 
function lengperpbisect(r1 , r2)
{
    var z = 2 * Math.sqrt((r1 * r1)
                        - ((r1 - r2)
                        * (r1 - r2) / 4));
 
    document.write("The length of the "
        + "perpendicular bisector is "
        + z.toFixed(5) );
}
 
// Driver code
 
var r1 = 5, r2 = 3;
lengperpbisect(r1, r2);
 
 
// This code is contributed by 29AjayKumar
</script>
Output: 
The length of the perpendicular bisector is 9.79796

 

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