TICH CHO MINH NHA

=(2+2^2+2^3+2^4+2^5)+.......................+(2^115+2^116+2^117+2^118+2^119+2^120)

=2(1+2+2^2+2^3+2^4+2^5)+............................+2^115(1+2+2^2+2^3+2^4+2^5)

=2.63+.....................+2^115.63

=63.(2+.............+2^115)

VÌ 63 CHIA HẾT CHO 21 NÊN A CHIA HẾT CHO 21