#include <cmath>
#include <iostream>

float solve(float (*)(float), float (*)(float), float, float);

float fx(float);
float dfx(float);

int main() {
    const float eps = 0.0001;

    float x1 = solve(fx, dfx, -5, eps),
    x2 = solve(fx, dfx, 0, eps),
    x3 = solve(fx, dfx, 2, eps);
    std::cout << "x1 = " << x1 << std::endl <<
    "x2 = " << x2 << std::endl <<
    "x3 = " << x3 << std::endl;
    std::cout << "f(x1) = " << fx(x1) << std::endl <<
    "f(x2) = " << fx(x2) << std::endl <<
    "f(x3) = " << fx(x3) << std::endl;

    return 0;
}

float solve(float (*fx)(float), float (*dfx)(float), float x0, const float eps) {
    float x = x0;
    while (std::abs(fx(x)) > eps) {
        if (dfx(x) == 0 || fx(x) * fx(x + eps * (x - fx(x) / dfx(x) - x > 0 ? 1.f : -1.f)) < 0) {
            break;
        }
        x -= fx(x) / dfx(x);
    }
    return x;
}

float fx(float x) {
    return (x * x * x) + (3 * x * x) + (-8 * x) + (1);
}

float dfx(float x) {
    return (3 * x * x) + (6 * x) + (-8);
}