#include <iostream>
#include <cmath>
#include <iomanip>
 
using namespace std;
 
int main() {
    double x;
    int n;
 
    cout << "Enter x: ";
    cin >> x;
    cout << "Enter number of terms (n): ";
    cin >> n;
 
    double sum = 0.0;
    double term = (x * x) / 2.0; // Перший член ряду при n=1: x^2 / 2!!
 
    for (int i = 1; i <= n; ++i) {
        sum += term;
 
        // Рекурентний перехід до наступного члена:
        // a(i+1) = a(i) * (x^2 / (2 * (i + 1)))
        term *= (x * x) / (2.0 * (i + 1));
    }
 
    cout << fixed << setprecision(10);
    cout << "Sum (O(n)): " << sum << endl;
 
    return 0;
}

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