grad

#include <iostream>
#include <cmath>

using namespace std;

struct Vector {
    double x, y, z;
    Vector() {};
    Vector(double a, double b, double c) : x(a), y(b), z(c) {}
};

double f(double x, double y, double z) { //функція u(x, y, z)
    double a = pow(x * x + 2 * y - 3 * z - 0.9, 2);
    double b = pow(2 * x - y * y + z - 1.7, 2);
    double c = pow(2 * x + 3 * y - z * z, 2);
    return (a + b + c);
}

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


int main() {
    Vector a(1.2, 1.3, 1.4);
    Vector grad = df(f, a);

    cout << grad.x << " " << grad.y << " " << grad.z << endl; //похідні

    return 0;
}