Updated

#include "pch.h"
#include <iostream>
#include <cmath> //sinus
#include <iomanip> //setprecision
using namespace std;

double f(double x) {
	return (x / (1.2 - sin(2 * x)));
}

double concave(double x1, double a, double b) { //1.21
	return ((-2)*(f(a)*(b - x1) + f(x1)*(a - b) + f(b)*(x1 - a)) / ((b - x1)*(a - b)*(x1 - a)));
}

double xTwo(double x1, double a, double b) { //1.22
	return ((f(a)*((b*b) - (x1*x1))
		+ f(x1)*((a*a) - (b*b))
		+ f(b)*((x1*x1) - (a*a))) / (
			2 * (f(a)*(b - x1)
				+ f(x1)*(a - b)
				+ f(b)*(x1 - a)))
		);
}

int main() {
	int expansion_factor = 4, i = 1;
	double accuracy = 0.01;
	double x1[100], x2[100], a[100], b[100], shift1, shift2;
	x1[0] = 0.8; //initial point > 0
	cout << f(0) << " < " << f(x1[0]) << "\n\n"; //so
	a[1] = (-1)*expansion_factor*x1[0];	x1[1] = 0;	b[1] = x1[0];
	cout << "i\ta\tx1\tx2\tb \tf(a)\tf(x1)\tf(x2)\tf(b)\toperation\n";
	do {
		while (concave(x1[i], a[i], b[i]) <= 0) { //1.21, 1.24
			cout << i << "\t" << setprecision(3) << a[i] << "\t" << setprecision(3) << x1[i] << "\t" << setprecision(3) << "-" << "\t" << setprecision(3) << b[i] << "\t" << setprecision(3) << f(a[i]) << "\t" << setprecision(3) << f(x1[i]) << "\t" << setprecision(3) << "-" << "\t" << setprecision(3) << f(b[i]);
			cout << "\tconcave:" << concave(x1[i], a[i], b[i]) << "\n";
			a[i] = x1[i];
			x1[i] = b[i];
			b[i] = b[i] + expansion_factor * (b[i] - x1[i]);
		}

		x2[i] = xTwo(x1[i], a[i], b[i]);

		if (x2[i] < a[i] || x2[i] > b[i]) { //1.25
			cout << i << "\t" << setprecision(3) << a[i] << "\t" << setprecision(3) << x1[i] << "\t" << setprecision(3) << x2[i] << "\t" << setprecision(3) << b[i] << "\t" << setprecision(3) << f(a[i]) << "\t" << setprecision(3) << f(x1[i]) << "\t" << setprecision(3) << f(x2[i]) << "\t" << setprecision(3) << f(b[i]);
			cout << "\tout of interval\n";
			shift1 = a[i];
			shift2 = x1[i];
			a[i] = x2[i];
			x1[i] = shift1;
			b[i] = shift2;
		}

		if (x2[i] < x1[i]) {
			cout << i << "\t" << setprecision(3) << a[i] << "\t" << setprecision(3) << x1[i] << "\t" << setprecision(3) << x2[i] << "\t" << setprecision(3) << b[i] << "\t" << setprecision(3) << f(a[i]) << "\t" << setprecision(3) << f(x1[i]) << "\t" << setprecision(3) << f(x2[i]) << "\t" << setprecision(3) << f(b[i]);
			cout << "\tx2<x1,swapped\n";
			shift1 = x2[i];
			x2[i] = x1[i];
			x1[i] = shift1;
		}

		cout << i << "\t" << setprecision(3) << a[i] << "\t" << setprecision(3) << x1[i] << "\t" << setprecision(3) << x2[i] << "\t" << setprecision(3) << b[i] << "\t" << setprecision(3) << f(a[i]) << "\t" << setprecision(3) << f(x1[i]) << "\t" << setprecision(3) << f(x2[i]) << "\t" << setprecision(3) << f(b[i]);

		if (f(x1[i]) >= f(x2[i])) { //1.11 - 1.13
			cout << "\t(a,x1) deleted\n";
			a[i + 1] = x1[i];
			b[i + 1] = b[i];
			x1[i + 1] = x2[i];
		}
		else { //1.15 - 1.18
			cout << "\t(x2,b) deleted\n";
			a[i + 1] = a[i];
			b[i + 1] = x2[i];
			x1[i + 1] = x1[i];
		}
		i++;
	} while (x2[i-1]-x1[i-1] > accuracy);
	cout << "\n(x2 - x1) < accuracy\n";
	return 0;
}