a) (111+112+113+.......+118)
=(11+112)+(113+114)+(115+116)+(117+118)
=(11+11.11)+(113+113.11)+(115+115.11)+(117+117.11)
=11.(1+11)+113.(11+1)+115.(1+11)+117.(1+11)
=11.12+113.12+115.12+117.12
=(11+113+115+117).12 chia hết cho 12
=>đpcm
b) =7+72+73+74
=(7+73)+(72+74)
=7.(1+72)+72.(1+72)
=7.50+72.50
=50.(7+72) chia hết cho 50
=> đpcm