[tex] x = 1\cdot 2+ 2\cdot 3+\dots +n\cdot (n+1) = \dfrac{n\cdot (n+1)\cdot (n+2)}{3}[/tex] Poti demostra formula asta prin inductie.
#include <iostream>
using namespace std;
int main(){
int n;
float x;
cout<<"Introduceti valoarea lui n:";cin>>n;
x = n*(n+1)*(n+2)/3;
cout<<"x = "<<x;
return 0;}