N=3+3^2+3^3+3^4+3^5...3^1991+3^1992+3^1993+3^1994+3^1995=
=3(1+3+3^2+3^3+3^4)+...+3^1991(1+3+3^2+3^3+3^4)=
=(1+3+3^2+3^3+3^4)(3+3^6+3^11+3^16+...+3^1986+3^1991)=
=121*(3+3^6+3^11+3^16+...+3^1986+3^1991)=m121, m=multiplu
N=m121, rezulta ca se divide cu 121