M = 1 + 3 + (32 + 33 + 34 ) +..... + (398 + 399 + 3100 )
M = 4 + 32 .13 + ..... +398 .13
= 13.(32 + 35 + ... + 398 ) + 4
=> M chia 13 dư 4
M = 1 + 3 + 32 + ... + 399 + 3100
M = 1 + ( 3 + 32 + 33 ) + .. + ( 398 + 399 + 3100 )
M = 1 + 3 . ( 1 + 3 + 32 ) + ... + 398 . ( 1 + 3 + 32 )
M = 1 + 3 . 13 + ... + 398 . 13 = 1 + 13 . ( 3 + ... + 398 ) : 13 dư 1