C/C++ 프로그램: n 제곱 빼기(n-1) 제곱을 n번째 항목으로 하는 시퀀스의 합을 계산합니다.

Tn = n2 - (n-1)2

Input: 229137999 Output: 218194447

=> 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

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