Gaussian Quad.

#include <iostream>
#include <fstream>
#include <iomanip>
#include <math.h>
using namespace std;

double f(double x) {
    return sqrt(2 * x + 1);
}

double gaussianQuad(double *nodes, double *weights, double a, double b, int n) {
    double res = 0;
    for (int i = 0; i < n; i++) res += f((b + a) / 2 + (b - a) / 2 * nodes[i]) * weights[i];
    res *= (b - a) / 2;
    return res;
}

int main() {
    ofstream fout;
    ifstream fin;
    fin.open("input.txt");
    fout.open("output.txt");
    int n;
    double a, b;
    fin >> a >> b;
    fin >> n;
    double *nodes = new double[n], *weights = new double[n];
    for (int i = 0; i < n; i++) fin >> weights[i] >> nodes[i];
    fout << setprecision(10) << gaussianQuad(nodes, weights, a, b, n);
    return 0;
}