Răspuns :
3/81 + 6/81 + 9/81 +....+ 237/81 + 240/81=
(3+6+9+...+237)/81+240/81=
3(1+2+3+...+79)/81+240/81=
pentru 1+2+3+...+79 se apica formula lui Gauss
3[79x(79+1):2] / 81+240/81=
3(79x80:2)/81+240/81=
(3x3160)/81+240/81=
(9480+240)/81=
9720/81=120
(3+6+9+...+237)/81+240/81=
3(1+2+3+...+79)/81+240/81=
pentru 1+2+3+...+79 se apica formula lui Gauss
3[79x(79+1):2] / 81+240/81=
3(79x80:2)/81+240/81=
(3x3160)/81+240/81=
(9480+240)/81=
9720/81=120
