b1:
#include<bits/stdc++.h>
using namespace std;
bool checknt(long long x){
    if(x<2) return 0;
    for(int i=2; i<=sqrt(x); i++)
        if(x%i==0) return 0;
    return 1;
    }
int tcs(long long x){
    int s=0;
    while(x>0){
        s=s+x%10;
        x=x/10;
    }
    return s;
}
int main() {
    freopen("NGTO2.INP","r",stdin);
    freopen("NGTO2.OUT","w",stdout);
    long long n;
    cin>>n;
    if(checknt(tcs(n))) cout<<"YES";
    else cout<<"NO";

}

b2:

#include<iostream>
using namespace std;
long long M,N;
int main(){
    freopen("BOICHUNG.INP", "r", stdin);
    freopen("BOICHUNG.OUT", "w", stdout);
    cin >> M >> N;
    for (int i = 1; i <= M*N; i++){
    if (i % M == 0 and i % N == 0){
        cout << i << " ";
  }
 }
 return 0;
}

b3:
#include <bits/stdc++.h>
using namespace std;
long long  a,b,i,n;
int main(){
    freopen("PSTOIGIAN.INP", "r", stdin);
    freopen("PSTOIGIAN.OUT", "w", stdout);
    ios::sync_with_stdio;
    cin.tie();
    cout.tie();
    cin>>a>>b;
    n=__gcd(a,b);
    a/=n;
    b/=n;
    cout<<a<<"/"<<b;
    return 0;
}

`*` Số nguyên tố 2

#include <bits/stdc++.h>

using namespace std;

bool check(long long n) {
    if (n < 2) return 0;
    for (long long i = 2; i * i <= n; i++) {
        if (n % i == 0) return 0;
    }
    return 1;
}

int main()
{
    freopen("NGTO2.inp", "r", stdin);
    freopen("NGTO2.out", "w", stdout);
    long long n, sum = 0;
    cin >> n;
    while (n) sum += n % 10, n /= 10;
    if (check(sum)) cout << "YES";
    else cout << "NO";
    return 0;
}

`*` Các bội chung

#include <bits/stdc++.h>

using namespace std;

int main()
{
    freopen("BOICHUNG.inp", "r", stdin);
    freopen("BOICHUNG.out", "w", stdout);
    int n, m;
    cin >> n >> m;
    int l = n * m / __gcd(n, m), r = n * m;
    for (int i = l; i <= r; i += l) {
        cout << i << ' ';
    }   
    return 0;
}

`*` Phân số tối giản

#include <bits/stdc++.h>

using namespace std;

int main()
{
    freopen("PSTOIGIAN.inp", "r", stdin);
    freopen("PSTOIGIAN.out", "w", stdout);
    int a, b;
    cin >> a >> b;
    int gcd = __gcd(a, b);
    a /= gcd;
    b /= gcd;
    cout << a << '/' << b;   
    return 0;
}