# C/C++ Program to Find the sum of Series with the n-th term as n^2 – (n-1)^2

There are many types of series in mathematics which can be solved easily in C programming. This program is to find the sum of following of series in C program.

Tn = n2 - (n-1)2

Find the sum of all of the terms of series as Sn mod (109 + 7) and,

Sn = T1 + T2 + T3 + T4 + ...... + Tn

Input: 229137999
Output: 218194447

### Explanation

Tn can be expressed as 2n-1 to get it

As we know ,

=> Tn = n2 - (n-1)2
=>Tn = n2 - (1 + n2 - 2n)
=>Tn = n2 - 1 - n2 + 2n
=>Tn = 2n - 1.
find ∑Tn.
∑Tn = ∑(2n – 1)
Reduce the above equation to,
=>∑(2n – 1) = 2*∑n – ∑1
=>∑(2n – 1) = 2*∑n – n.
here, ∑n is the sum of first n natural numbers.
As known the sum of n natural number ∑n = n(n+1)/2.
Now the equation is,
∑Tn = (2*(n)*(n+1)/2)-n = n2
The value of n2 can be large. Instead of using n2 and take the mod of the result.
So, using the property of modular multiplication for calculating n2:
(a*b)%k = ((a%k)*(b%k))%k

## Example

#include <iostream>
using namespace std;
#define mod 1000000007
int main() {
long long n = 229137999;
cout << ((n%mod)*(n%mod))%mod;
return 0;
}