B=2+2^2+2^3+.......+2^30
B=(2+2^2+2^3)+(2^4+2^5+2^6)+......................+(2^28+2^29+2^30)
B=2x(1+2+2^2)+2^4x(1+2+2^2)+......+2^28 x (1+2+2^2)
B= 2x7+2^4x7+....................+2^28x7
B=7 x (2+2^4+..........+2^28)
Ta thấy 7chia hết cho 7 do đó 7 x (2+2^4+.....+2^28) cũng chia hết cho 7 hay B chia hết cho 7