A=2+2^2+2^3+......+2^29+2^30

A=(2+ 2^2 +2^3 +2^4+2^5 +2^6)+......+(2^25 +2^26 +2^27+ 2^28 +2^29 +2^30)

A=(2.1+ 2.2+ 2.2^2+ 2.2^3+ 2.2^4+ 2.2^5)+......+(2^25.1 +2^25.2 +2^25.2^2 +2^25.2^3 +2^25.2^4 +2^25.2^5)

A=2.(1+2+2^2+2^3+2^4+2^5)+......+2^25.(1+2+2^2+2^3+2^4+2^5)

A=2.(1+2+4+8+16+32)+.....+2^25.(1+2+4+8+16+32)

A=2.63+........+2^25.63

\(\Rightarrow\)A=63.(2+.....+2^25)

Vì 63:21=3 nên 63 chia hết cho 21

 \(\Rightarrow\)A=2+2^2+2^3+...+2^29+2^30 = 63.(2+......+2^25)  chia hết cho 21

 Vậy : Tổng A chia hết cho 21

Bạn C nha k minh yêu