A= 20 + 21+ 22 +23 + ....+ 2100=(20+21+22+23)+(24+25+26+27)+...+(296+297+298+299)+2100
=15+24.(1+2+22+23)+...+296.(1+2+22+23)+2100
=15+24.15+...+296.15+2100
=15.(1+24+...+296)+2100
Vậy A chia cho 15 dư 2100
Ta có: A = 1 + ( 2 + 22 + 23 + 24 ) + ... + 296( 2 + 22 + 23 + 24 )
A = 1 + 30 + ... + 296 . 30
A = 1 + 30( 1 + 24 + ... + 296 )
Mà 30 chia hết cho 15 nên 30( 1 + 24 + ... + 296 ) chia hết cho 15
\(\Rightarrow\) 1 + 30( 1 + 24 + ... + 296 ) : 15 dư 1
\(\Rightarrow\) A : 15 dư 1
a=20+....+2100
a=1+( 2 + 4+8+16) + ....+ 296(2+4+8+16)
vì 2+4+8+16= 21+22+23+24.mà 2+4+8+16 =30
30 chiahet 15 suy ra 1 +.....chia 15 dư 1
1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Ta có:
A= 20 + 21 + 22 + 23 + ... + 2100
= 1 + 21 + 22 + 23 + ... + 2100
= 1 + (21 + 22 + 23 + 24) + ... + 2100
= 1 + (21 + 22 + 23 + 24) + (25 + 26 + 27 + 28) + ... + 290 x 210
= 1 + (21 + 22 + 23 + 24) + 24(21 + 22 + 23 + 24) + ... + 290(21 + 22 + 23 + 24)
= 1 + 30 + 24 x 30 + ... + 290 x 30
= 1 + 30(1 + 24 + ... + 290)
Vì: 30 chia hết cho 15. Suy ra: 30(1 + 24 + ... + 290) chia hết cho 15
Suy ra: 1 + 30(1 + 24 + ... + 290) chia cho 15 dư 1
Vậy: A= 20 + 21 + 22 + 23 + ... + 2100 chia cho 15 dư 1