A=(1+2+2^2+2^3)+.....+(2^16+2^17+2^18+2^19)
A=1(1+2+2^2+2^3)+....+2^16(1+2+2^2+2^3)
A=1.15+.....+2^16.15
A=15(1+.....+2^16)
Suy ra A chia hết cho 15
Ta có:A=1+2+22+23+.....+219
=(1+2+22+23)+...+(216+217+218+219)
=15+....+216.15
=15.(1+.....+216) chia hết cho 15
Vậy đpcm
A = 1 + 2 + 22 + 23 + ... + 219
A = ( 1 + 2 + 22 + 23 ) + ...+ ( 216 + 217 + 218 + 219 )
A = 15 + ... + 216 ( 1 + 2 + 22 + 23 )
A = 15 + ... + 216 . 15
A = 15 ( 1 + 24 + ... + 216 ) \(⋮\)15
A=(1+2+22+23)+(24+25+26+27)+....+(216+217+218+219)
A=(1+2+22+23)+24(1+2+22+23)+...+216(1+2+22+23)
A=(1+2+22+23)(1+24+...+216)
A=15.(1+24+...+216) chia hết cho 15