3 lab

#include <iostream>
#include <cmath>

using namespace std;

struct Vector {
    double x, y, z;
    Vector() {x = y = z = 0.;}
    Vector(double a, double b, double c) : x(a), y(b), z(c) {}
    Vector operator * (const double k) {
        Vector c(x * k, y * k, z * k);
        return c;
    }
    Vector operator - (const Vector &a) {
        Vector c(x - a.x, y - a.y, z - a.z);
        return c;
    }
};

double f(double x, double y, double z) { // u(x, y, z)
    double a = 4 * x - 5 * pow(y, 2) + z - 3.75;
    double b = 3 * pow(x, 2) + 2 * y - 2 * z - 3;
    double c = x + y + 2 * pow(z, 2) - 3.5;
    return pow(a, 2) + pow(b, 2) + pow(c, 2);
}

Vector df(double f(double x, double y, double z), Vector a) {
    const double d = 1e-10;
    Vector grad;

    grad.x = (f(a.x + d, a.y, a.z) - f(a.x, a.y, a.z)) / d;
    grad.y = (f(a.x, a.y + d, a.z) - f(a.x, a.y, a.z)) / d;
    grad.z = (f(a.x, a.y, a.z + d) - f(a.x, a.y, a.z)) / d;
    return grad;
}

double kNorma(Vector a) {
    return sqrt(pow(a.x, 2)) + (pow(a.y, 2)) + (pow(a.z, 2));
}

void solve(double u(double x, double y, double z), double EPS, double L) {
    Vector prevX, x;
    int i = 0;
    double d;

    do {
        prevX = x;
        d = kNorma(df(u, x));
        x = prevX - df(u, prevX) * L;

        cout << "Iteration :: " << i << "\t   >>\tx = {" << x.x << ", " << x.y << ", " << x.z << "}";
        cout << "\t   >>\td = " << d;
        if (d < EPS) cout << " < EPS";
        cout << endl;

        i++;
    } while (d >= EPS);
    return;
}

int main() {

    solve(f, 1e-4, 1e-3);

    return 0;
}