a)Ta có: C=2+22+23+...+2100 C=1.(2+22+23+24+25)+25.(2+22+23+24+25)+...+295.(2+22+23+24+25) C=2.31+25.2.31+...+295.2.31 C=31(2+2.25+...+2.295) =>C chia hết cho 31
a)C=2+22+23+...+2100
=2.(1+2+4+8+16)+26.(1+2+4+8+16)+...+296.+(1+2+4+8+16)
=2.31+26.31+...+296.31
=31.(2+26+...+296) chia hết cho 31
C chia hết cho 31
Giaỉ
a, C = 2 + 22 + 23 + ... + 2100
C = ( 2 + 22 + 23 + 24 + 25 ) + ( 26 + 27 + 28 + 29 + 210 ) + ... + ( 296 + 297 + 298 + 299 + 2100 )
C = 2( 1 + 2 + 22 + 23 + 24 ) + 26( 1 + 2 + 22 + 23 + 24 ) + ... + 296( 1 + 2 + 22 + 23 + 24 )
C = 2 . 31 + 26 . 31 + ... + 296 . 31
C = 31( 2 + 26 +... + 296 ) chia hết cho 31
=> C chia hết cho 31.
b, C = 2 + 22 + 23 + ... + 2100
2 x C = 22 + 23 + 24 + ... + 2101
2 x C - C = ( 22 + 23 + 24 + ... + 2101 ) - ( 2 + 22 + 23 + ... + 2100 )
C = ( 2101 - 2 ) : 2