double FuncMeanSquare(double x, double* c)
	{
		return c[0] + c[1] * x + c[2] * pow(x, 2);
	}

	double scalarFF = 0; // (f,f)
	double scalarFG = 0; // (f,g[i])

	for (int i = 0; i < n; i++)
	{
		scalarFF += pow(F(xArr1[i]), 2);
		scalarFG += pow(FuncMeanSquare(xArr1[i], c), 2);
	}

	double error = sqrt(scalarFF - scalarFG);