=> A = ( 1 + 5 + 52 ) + ( 53 + 54 + 55 ) + ... + ( 5402 + 5403 + 5404 )
=> A = 1.( 1 + 5 + 5.5 ) + 53.( 1 + 5 + 5.5 ) + .... + 5402.( 1 + 5 + 5.5 )
=> A = 1.31 + 53.31 + .... + 5402.31
=> A = 31.( 1 + 53 + .... + 5402 )
Vì 31 ⋮ 31 nên A ⋮ 31 ( đpcm )
A=1+5+52+53+...+5403+5404CMR A chia hết cho 31
A=1+5+52+53+...+5403+5404
\(\Rightarrow\)5*A = 5( 1+5+52+53+...+5403+5404 )
= 5+52+53+...+5403+5404+5405
\(\Rightarrow\)4*A = ( 5+52+53+...+5403+5404+5405 ) - ( 1+5+52+53+...+5403+5404 )
= 5405 -1
\(\Rightarrow\)A = ( 5405 -1 ) :4
