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Program to check if N is a Pentagonal Number in C++
Given with a number N the task is to check whether the number is a pentagonal number or not. Numbers that can be arranged to form a pentagon is a pentagonal number as these numbers can be used as points to form a pentagon. For example, some of pentagonal numbers are 1, 5, 12, 22, 35, 51....
We can use formula to check whether the number is a pentagonal number or not
$$p(n)=\frac{\text{3}*n^2-n}{\text{2}}$$
Where, n is the number of points pentagonal will have
Example
Input-: n=22 Output-: 22 is pentagonal number Input-: n=23 Output-: 23 is not a pentagonal number
Algorithm
Start Step 1 -> declare function to Check N is pentagonal or not bool check(int n) declare variables as int i = 1, a do set a = (3*i*i - i)/2 set i += 1 while ( a < n ); return (a == n); Step 2 -> In main() Declare int n = 22 If (check(n)) Print is pentagonal End Else Print it is not pentagonal End Stop
Example
#include <iostream>
using namespace std;
// check N is pentagonal or not.
bool check(int n){
int i = 1, a;
do{
a = (3*i*i - i)/2;
i += 1;
}
while ( a < n );
return (a == n);
}
int main(){
int n = 22;
if (check(n))
cout << n << " is pentagonal " << endl;
else
cout << n << " is not pentagonal" << endl;
return 0;
}Output
22 is pentagonal
Updated on: 23-Sep-2019
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