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C program to find Decagonal Number

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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…… decagonal number Examples:

Input : n = 2
Output : 10

Input : n = 5
Output : 85

Input : n = 7
Output: 175


// C program to find nth decagonal number
#include <stdio.h>
#include <stdlib.h>
// 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",
    return 0;


Decagonal Number is = 370

Time complexity: O(1) as constant operations are done

Auxiliary space: O(1)

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 or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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Last Updated : 19 Sep, 2022
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