# C program to find Decagonal Number

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

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