Đặt A=3^0+3^1+3^2+3^3+...+3^11

=>A=(3^0+3^1+3^2+3^3)+(3^4+3^5+3^6+3^7)+(3^8+3^9+3^10+3^11)

=>A=40+3^4(1+3+3^2+3^3)+3^8(1+3+3^2+3^3)

=>A=40+3^4.40+3^8.40

=>A=40(1+3^4+3^8)

=>A chia hết cho 40

Vậy 3^0+3^1+3^2+3^3+...+3^11 chia hết cho 40