Calculați suma Sn=3+33+333+...+333...3, n aparține N*

Răspuns :

Sn= 3+ 3*10+3+ 3*100+ 3*10+ 3+...
Sn=3*n+ 3*(n-1)*10^1+ 3*(n-2)*10^2+....+3*(n-x)*10^x (n>x, n!=x)