Radius of the inscribed circle within three tangent circles
There are 4 circles with positive integer radius r1, r2, r3 and r4 as shown in the figure below.
The task is to find the radius r4 of the circle formed by three circles when radius r1, r2, r3 are given.
(Note that the circles in the picture above are tangent to each other.)
Examples:
Input: r1 = 1, r2 = 1, r3 = 1
Output: 0.154701Input: r1 = 23, r2 = 46, r3 = 69
Output: 6.000000
Approach 1: (Using Binary Search) :
- The policy is to join the centers of all the circles and form 4 triangles
- After the triangles are formed, equate the sum of areas of the three smaller triangles with the main triangle as far as possible using binary search.
Analysis of the mentioned approach:
- This method works because initially there are 4 triangles as pointed in the above image.
- The main triangle with sides and the three smaller triangles with sides .
- The main triangle consists of the small ones so the area of the main triangle is the sum of the areas of the smaller ones.
Forming a search space:
Here binary search. The value for r can be chosen and the sum of the areas of all three smaller triangles can be computed and compared with the area of the main triangle.
- Choosing lower bound
- Choosing upper bound
By intuition, the upper bound value for r4 as the radius of the inscribed circle into the triangle is less than:
rupper_bound
Now Binary Search can be applied at the following search space.
Below is the implementation of the problem using the above approach.
C++
// C++ implementation of the approach #include <bits/stdc++.h> using namespace std; // Radius of the 3 given circles // declared as double. double r1, r2, r3; // Calculation of area of a triangle by Heron's formula double area( double a, double b, double c) { double p = (a + b + c) / 2; return sqrt (p) * sqrt (p - a) * sqrt (p - b) * sqrt (p - c); } // Applying binary search to find the // radius r4 of the required circle double binary_search() { // Area of main triangle double s = area(r1 + r2, r2 + r3, r3 + r1); double l = 0, h = s / (r1 + r2 + r3); // Loop runs until l and h becomes approximately equal while (h - l >= 1.e-7) { double mid = (l + h) / 2; // Area of smaller triangles double s1 = area(mid + r1, mid + r2, r1 + r2); double s2 = area(mid + r1, mid + r3, r1 + r3); double s3 = area(mid + r2, mid + r3, r2 + r3); // If sum of smaller triangles // is less than main triangle if (s1 + s2 + s3 < s) { l = mid; } // If sum of smaller triangles is // greater than or equal to main triangle else { h = mid; } } return (l + h) / 2; } // Driver code int main() { // Taking r1, r2, r3 as input r1 = 1.0; r2 = 2.0; r3 = 3.0; // Call to function binary search cout << fixed << setprecision(6) << binary_search() << endl; return 0; } |
Java
// Java implementation of the approach import java.util.*; class GFG { // Radius of the 3 given circles // declared as double. static double r1, r2, r3; // Calculation of area of a triangle by Heron's formula static double area( double a, double b, double c) { double p = (a + b + c) / 2 ; return Math.sqrt(p) * Math.sqrt(p - a) * Math.sqrt(p - b) * Math.sqrt(p - c); } // Applying binary search to find the // radius r4 of the required circle static double binary_search() { // Area of main triangle double s = area(r1 + r2, r2 + r3, r3 + r1); double l = 0 , h = s / (r1 + r2 + r3); // Loop runs until l and h becomes approximately equal while (h - l >= 1 .e- 7 ) { double mid = (l + h) / 2 ; // Area of smaller triangles double s1 = area(mid + r1, mid + r2, r1 + r2); double s2 = area(mid + r1, mid + r3, r1 + r3); double s3 = area(mid + r2, mid + r3, r2 + r3); // If sum of smaller triangles // is less than main triangle if (s1 + s2 + s3 < s) { l = mid; } // If sum of smaller triangles is // greater than or equal to main triangle else { h = mid; } } return (l + h) / 2 ; } // Driver code public static void main(String[] args) { // Taking r1, r2, r3 as input r1 = 1.0 ; r2 = 2.0 ; r3 = 3.0 ; // Call to function binary search System.out.printf( "%.6f" , binary_search()); } } // This code is contributed by 29AjayKumar |
Python3
# Python3 implementation of the approach import math # Radius of the 3 given circles r1 = 0 r2 = 0 r3 = 0 # Calculation of area of a # triangle by Heron's formula def area(a, b, c): p = (a + b + c) / 2 return ((math.sqrt(p)) * (math.sqrt(p - a)) * (math.sqrt(p - b)) * (math.sqrt(p - c))) # Applying binary search to find the # radius r4 of the required circle def binary_search(): global r1, r2, r3 # Area of main triangle s = area(r1 + r2, r2 + r3, r3 + r1) l = 0 h = s / (r1 + r2 + r3) # Loop runs until l and h # becomes approximately equal while (h - l > 0.00000001 ): mid = (l + h) / 2 # Area of smaller triangles s1 = area(mid + r1, mid + r2, r1 + r2) s2 = area(mid + r1, mid + r3, r1 + r3) s3 = area(mid + r2, mid + r3, r2 + r3) # If sum of smaller triangles # is less than main triangle if (s1 + s2 + s3 < s): l = mid # If sum of smaller triangles is # greater than or equal to main triangle else : h = mid return ((l + h) / 2 ) # Driver code # Taking r1, r2, r3 as input r1 = 1 r2 = 2 r3 = 3 # Call to function binary search print ( "{0:.6f}" . format (binary_search())) # This code is contributed by avanitrachhadiya2155 |
C#
// C# implementation of the approach using System; class GFG { // Radius of the 3 given circles // declared as double. static double r1, r2, r3; // Calculation of area of a triangle by Heron's formula static double area( double a, double b, double c) { double p = (a + b + c) / 2; return Math.Sqrt(p) * Math.Sqrt(p - a) * Math.Sqrt(p - b) * Math.Sqrt(p - c); } // Applying binary search to find the // radius r4 of the required circle static double binary_search() { // Area of main triangle double s = area(r1 + r2, r2 + r3, r3 + r1); double l = 0, h = s / (r1 + r2 + r3); // Loop runs until l and h // becomes approximately equal while (h - l > 0.00000001) { double mid = (l + h) / 2; // Area of smaller triangles double s1 = area(mid + r1, mid + r2, r1 + r2); double s2 = area(mid + r1, mid + r3, r1 + r3); double s3 = area(mid + r2, mid + r3, r2 + r3); // If sum of smaller triangles // is less than main triangle if (s1 + s2 + s3 < s) { l = mid; } // If sum of smaller triangles is // greater than or equal to main triangle else { h = mid; } } return (l + h) / 2; } // Driver code public static void Main(String[] args) { // Taking r1, r2, r3 as input r1 = 1.0; r2 = 2.0; r3 = 3.0; // Call to function binary search Console.Write( "{0:F6}" , binary_search()); } } // This code is contributed by Rajput-Ji |
Javascript
<script> // Javascript implementation of the approach // Radius of the 3 given circles // declared as double. let r1, r2, r3; // Calculation of area of a triangle // by Heron's formula function area(a, b, c) { let p = (a + b + c) / 2; return Math.sqrt(p) * Math.sqrt(p - a) * Math.sqrt(p - b) * Math.sqrt(p - c); } // Applying binary search to find the // radius r4 of the required circle function binary_search() { // Area of main triangle let s = area(r1 + r2, r2 + r3, r3 + r1); let l = 0, h = s / (r1 + r2 + r3); // Loop runs until l and h // becomes approximately equal while (h - l >= 1.e-7) { let mid = (l + h) / 2; // Area of smaller triangles let s1 = area(mid + r1, mid + r2, r1 + r2); let s2 = area(mid + r1, mid + r3, r1 + r3); let s3 = area(mid + r2, mid + r3, r2 + r3); // If sum of smaller triangles // is less than main triangle if (s1 + s2 + s3 < s) { l = mid; } // If sum of smaller triangles is // greater than or equal to main triangle else { h = mid; } } return (l + h) / 2; } // Driver code // Taking r1, r2, r3 as input r1 = 1.0; r2 = 2.0; r3 = 3.0; // Call to function binary search document.write(binary_search().toPrecision(6)); // This code is contributed by subhammahato348 </script> |
Output:
0.260870
Time Complexity: O(logn)
Auxiliary Space: O(1)
Approach 2: (Using Descartes’ Theorem)
- According to Descartes’ Theorem, the reciprocals of radii, or “curvatures”, of these circles satisfy the following relation.
- If are known, one can solve for,
- On solving the above equation;
Below is the implementation of the problem using the above formula.
C++
// C++ implementation of the approach #include <bits/stdc++.h> using namespace std; // Driver code int main() { // Radius of the 3 given circles declared as double. double r1, r2, r3; // Taking r1, r2, r3 as input r1 = 1; r2 = 2; r3 = 3; // Calculation of r4 using formula given above double r4 = (r1 * r2 * r3) / (r1 * r2 + r2 * r3 + r1 * r3 + 2.0 * sqrt (r1 * r2 * r3 * (r1 + r2 + r3))); cout << fixed << setprecision(6) << r4 << '\n' ; return 0; } |
Java
// Java implementation of the approach class GFG { // Driver code public static void main(String[] args) { // Radius of the 3 given circles declared as double. double r1, r2, r3; // Taking r1, r2, r3 as input r1 = 1 ; r2 = 2 ; r3 = 3 ; // Calculation of r4 using formula given above double r4 = (r1 * r2 * r3) / (r1 * r2 + r2 * r3 + r1 * r3 + 2.0 * Math.sqrt(r1 * r2 * r3 * (r1 + r2 + r3))); System.out.printf( "%.6f" , r4); } } // This code is contributed by 29AjayKumar |
Python3
# Python3 implementation of the approach from math import sqrt # Driver code # Radius of the 3 given circles declared as double. # Taking r1, r2, r3 as input r1 = 1 r2 = 2 r3 = 3 # Calculation of r4 using formula given above r4 = (r1 * r2 * r3) / (r1 * r2 + r2 * r3 + r1 * r3 + 2.0 * sqrt(r1 * r2 * r3 * (r1 + r2 + r3))) print ( round (r4, 6 )) # This code is contributed by mohit kumar 29 |
C#
// C# implementation of the approach using System; class GFG { // Driver code public static void Main(String[] args) { // Radius of the 3 given circles declared as double. double r1, r2, r3; // Taking r1, r2, r3 as input r1 = 1; r2 = 2; r3 = 3; // Calculation of r4 using formula given above double r4 = (r1 * r2 * r3) / (r1 * r2 + r2 * r3 + r1 * r3 + 2.0 * Math.Sqrt(r1 * r2 * r3 * (r1 + r2 + r3))); Console.Write( "{0:F6}" , r4); } } // This code contributed by PrinciRaj1992 |
Javascript
<script> // Javascript implementation of the approach // Driver code // Radius of the 3 given circles // declared as double. let r1, r2, r3; // Taking r1, r2, r3 as input r1 = 1; r2 = 2; r3 = 3; // Calculation of r4 using formula given above let r4 = (r1 * r2 * r3) / (r1 * r2 + r2 * r3 + r1 * r3 + 2.0 * Math.sqrt( r1 * r2 * r3 * (r1 + r2 + r3))); document.write(r4.toFixed(6) + "<br>" ); // This code is contributed by souravmahato348 </script> |
Output:
0.260870
Time Complexity: O(log(n)) as inbuilt sqrt function is being used which has time complexity of log(n)
Auxiliary Space: O(1)
Reference :
- https://brilliant.org/wiki/descartes-theorem/
- https://en.wikipedia.org/wiki/Descartes%27_theorem/
- http://www.ambrsoft.com/TrigoCalc/Circles3/Tangency/Tangent.htm