Verificam P(1) :
1=(1x(1+1))/2
1=2/2
1=1
Presupunem adevarat P(k):
1+2+...+k=(k(k+1))/2
Demonstram P(k+1)
1+2+3+....+k+(k+1)=((k+1)(k+2))/2
(k(k+1))/2+(k+1)=(k(k+1)+2(k+1))/2=((k+1)(k+2))/2
Rezulta P(n) adevarat
