# 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 = 1Output:0.154701

Input:r1 = 23, r2 = 46, r3 = 69Output: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:

r_{upper_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

**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

**Reference** :

- https://brilliant.org/wiki/descartes-theorem/
- https://en.wikipedia.org/wiki/Descartes%27_theorem/
- http://www.ambrsoft.com/TrigoCalc/Circles3/Tangency/Tangent.htm

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