a, S = 3 + 32 + 33 + .... + 31998
= ( 3 + 32 ) + ( 33 + 34 ) +......+ ( 31997 + 31998 )
= 12 + 33( 3 + 32 ) + ..... + 31997( 3 + 32 )
= 12 + 33 . 12 + ...... + 31997 . 12
= 12 . ( 1 + 33 + ..... + 31997 ) chia hết cho 12
=> S chia hết cho 12
b, S = 3 + 32 + 33 + .... + 31998
= ( 3 + 32 + 33 ) + ...... + ( 31996 + 31997 + 31998 )
= 39 + 34( 3 + 32 + 33 ) + ..... + 31996( 3 + 32 + 33 )
= 39 + 34 . 39 + ........ + 31996 . 39
= 39( 1 + 34 + ..... + 31996 ) chia hết cho 39
=> S chia hết cho 39
a,S=3+32+33+34+.........+31998
S=(3+32)+(33+34)+........+(31997+31998)
S=12+32.(3+32)+............+31996.(3+32)
S=12+32.12+.......+31996.12
S=(1+32+..........+31996).12 chia hết cho 12(đpcm)
b,S=(3+32+33)+(34+35+36)+..........+(31996+31997+31998)
S=39+33.(3+32+33)+............+31995.(3+32+33)
S=39+33.39+...........+31995.39
S=(1+33+........+31995).39 chia hết cho 39(đpcm)
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