Given two integers **N **and **K** where N denotes the unit size of a bigger Equilateral Triangle, the task is to find the number of an equilateral triangle of size K that are present in the bigger triangle of side N.

**Examples:**

Input:N = 4, K = 3

Output:3Explanation:

There are 3 equilateral triangles of 3 unit size which are present in the Bigger equilateral triangle of size 4 units.

Input:N = 4, K = 2Output:7Explanation:

There are 7 equilateral triangles of 2 unit size which are present in the Bigger equilateral triangle of size 4 units.

**Naive Approach: **The idea is to iterate over all possible sizes of the bigger equilateral triangle for checking the number of triangles with the required size **K** and print the total count of triangles.

**Time Complexity:** O(N)**Auxiliary Space:** O(1)

**Efficient Approach:** To optimize the above approach, observe the following points:

- The number of triangles with a peak in the upward direction of size
**K**present in size**N**equals to**((N – K +1 ) * (N – K + 2))/2**. - The number of inverted triangles with a peak in the downward direction of size
**K**present in size**N**equals to**((N – 2K + 1) * (N – 2K + 2))/2**.

Below is the implementation of the above approach:

## C++

`// C++ program for the above approach` `#include <iostream>` `using` `namespace` `std;` `// Function to find the number of` `// equilateral triangle formed` `// within another triangle` `int` `No_of_Triangle(` `int` `N, ` `int` `K)` `{` ` ` `// Check for the valid condition` ` ` `if` `(N < K)` ` ` `return` `-1;` ` ` `else` `{` ` ` `int` `Tri_up = 0;` ` ` `// Number of triangles having` ` ` `// upward peak` ` ` `Tri_up = ((N - K + 1)` ` ` `* (N - K + 2))` ` ` `/ 2;` ` ` `int` `Tri_down = 0;` ` ` `// Number of inverted triangles` ` ` `Tri_down = ((N - 2 * K + 1)` ` ` `* (N - 2 * K + 2))` ` ` `/ 2;` ` ` `// Total no. of K sized triangle` ` ` `return` `Tri_up + Tri_down;` ` ` `}` `}` `// Driver Code` `int` `main()` `{` ` ` `// Given N and K` ` ` `int` `N = 4, K = 2;` ` ` `// Function Call` ` ` `cout << No_of_Triangle(N, K);` ` ` `return` `0;` `}` |

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## Java

`// Java program for the above approach` `import` `java.util.*;` `class` `GFG{` `// Function to find the number of` `// equilateral triangle formed` `// within another triangle` `static` `int` `No_of_Triangle(` `int` `N, ` `int` `K)` `{` ` ` `// Check for the valid condition` ` ` `if` `(N < K)` ` ` `return` `-` `1` `;` ` ` `else` ` ` `{` ` ` `int` `Tri_up = ` `0` `;` ` ` `// Number of triangles having` ` ` `// upward peak` ` ` `Tri_up = ((N - K + ` `1` `) * (N - K + ` `2` `)) / ` `2` `;` ` ` `int` `Tri_down = ` `0` `;` ` ` `// Number of inverted triangles` ` ` `Tri_down = ((N - ` `2` `* K + ` `1` `) * ` ` ` `(N - ` `2` `* K + ` `2` `)) / ` `2` `;` ` ` `// Total no. of K sized triangle` ` ` `return` `Tri_up + Tri_down;` ` ` `}` `}` `// Driver Code` `public` `static` `void` `main(String[] args)` `{` ` ` `// Given N and K` ` ` `int` `N = ` `4` `, K = ` `2` `;` ` ` `// Function Call` ` ` `System.out.print(No_of_Triangle(N, K));` `}` `}` `// This code is contributed by PrinciRaj1992` |

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## Python3

`# Python3 program for the above approach` `# Function to find the number of` `# equilateral triangle formed` `# within another triangle` `def` `No_of_Triangle(N, K):` ` ` ` ` `# Check for the valid condition` ` ` `if` `(N < K):` ` ` `return` `-` `1` `;` ` ` `else` `:` ` ` `Tri_up ` `=` `0` `;` ` ` `# Number of triangles having` ` ` `# upward peak` ` ` `Tri_up ` `=` `((N ` `-` `K ` `+` `1` `) ` `*` ` ` `(N ` `-` `K ` `+` `2` `)) ` `/` `/` `2` `;` ` ` `Tri_down ` `=` `0` `;` ` ` `# Number of inverted triangles` ` ` `Tri_down ` `=` `((N ` `-` `2` `*` `K ` `+` `1` `) ` `*` ` ` `(N ` `-` `2` `*` `K ` `+` `2` `)) ` `/` `/` `2` `;` ` ` `# Total no. of K sized triangle` ` ` `return` `Tri_up ` `+` `Tri_down;` ` ` `# Driver Code` `if` `__name__ ` `=` `=` `'__main__'` `:` ` ` `# Given N and K` ` ` `N ` `=` `4` `; K ` `=` `2` `;` ` ` `# Function Call` ` ` `print` `(No_of_Triangle(N, K));` `# This code is contributed by sapnasingh4991` |

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## C#

`// C# program for the above approach` `using` `System;` `class` `GFG{` `// Function to find the number of` `// equilateral triangle formed` `// within another triangle` `static` `int` `No_of_Triangle(` `int` `N, ` `int` `K)` `{` ` ` `// Check for the valid condition` ` ` `if` `(N < K)` ` ` `return` `-1;` ` ` `else` ` ` `{` ` ` `int` `Tri_up = 0;` ` ` `// Number of triangles having` ` ` `// upward peak` ` ` `Tri_up = ((N - K + 1) * (N - K + 2)) / 2;` ` ` `int` `Tri_down = 0;` ` ` `// Number of inverted triangles` ` ` `Tri_down = ((N - 2 * K + 1) * ` ` ` `(N - 2 * K + 2)) / 2;` ` ` `// Total no. of K sized triangle` ` ` `return` `Tri_up + Tri_down;` ` ` `}` `}` `// Driver Code` `public` `static` `void` `Main(String[] args)` `{` ` ` `// Given N and K` ` ` `int` `N = 4, K = 2;` ` ` `// Function Call` ` ` `Console.Write(No_of_Triangle(N, K));` `}` `}` `// This code is contributed by Rajput-Ji` |

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**Output:**

7

**Time Complexity:** O(1)**Auxiliary Space:** O(1)

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