=> E = ( 7 + 72 ) + ( 73 + 74 ) + .... + ( 733 + 734 ) + ( 735 + 736 )
=> E = 7.( 1 + 7 ) + 73.( 1 + 7 ) + .... + 733.( 1 + 7 ) + 735.( 1 + 7 )
=> E = 7.8 + 73.8 + .... + 733.8 + 735.8
=> E = 8.( 7 + 73 + .... + 733 + 735 )
Vì 8 ⋮ 8 nên E chia 8 dư 0