C++ program to find Nth number of the series 1, 6, 15, 28, 45, .....

In this problem, we are given an integer value N. Our task is to create a program to Find Nth number of the series 1, 6, 15, 28, 45, …

In the series, every element is 2 less than the mean of the previous and next element.

Let’s take an example to understand the problem,

Input

N = 5

Output

45

Solution Approach

The Nth term of the series 1, 6, 15, 28, 45, … can be found using the formula,

T<sub>N</sub> = 2*N*N - N

Program to illustrate the working of our solution,

Example

#include <iostream>
using namespace std;
#define mod 1000000009
int calcNthTerm(long n) {
   return (((2 * n * n) % mod) - n + mod) % mod;
}
int main(){
   long N = 19;
   cout<<N<<"th Term of the series is "<<calcNthTerm(N);
   return 0;
}

Output

19th Term of the series is 703