Related Articles

# C program to find Decagonal Number

• Last Updated : 04 Dec, 2018

A decagonal number is a figurate number that extends the concept of triangular and square numbers to the decagon (a ten-sided polygon). The nth decagonal numbers counts the number of dots in a pattern of n nested decagons, all sharing a common corner, where the ith decagon in the pattern has sides made of i dots spaced one unit apart from each other. The n-th decagonal number is given by the formula D(n)=4n2-3n;
The first few decagonal numbers are:
0, 1, 10, 27, 52, 85, 126, 175, 232, 297, 370, 451, 540, 637, 742, 855, 976, 1105, 1242……

```Input : n = 2
Output : 10

Input : n = 5
Output : 85

Input : n = 7
Output: 175
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

 `// C program to find nth decagonal number``#include ``#include `` ` `// Finding the nth  Decagonal Number``int` `decagonalNum(``int` `n)``{``    ``return` `(4 * n * n - 3 * n);``}`` ` `// Driver program to test above function``int` `main()``{``    ``int` `n = 10;``    ``printf``(``"Decagonal Number is = %d"``,``           ``decagonalNum(n));`` ` `    ``return` `0;``}`
Output:
```Decagonal Number is = 370
```

References : Mathworld

This article is contributed by DANISH_RAZA. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.