Ta có:

Ư(4) = { 1, 2, 4 }

Ư(6) = { 1, 2, 3, 6 }

Ư(8) = { 1, 2, 4, 8 }

Ư(12) = { 1, 2, 3, 4, 6, 12 }

Ư(18) = { 1, 2, 3, 6, 9, 18 }

B(4) = {0, 4, 8, 12,24,.}

B(6) = { 0, 6, 12, 18,24,..}

B(8) = { 0, 8, 16, 24, ..}

B(20) = 0,20, 40,60,... }

B(30) = { 0, 30, 60,... }

=>  Ư(12,18) = { 1, 2, 3, 6 }

       ƯC(4,6,8) = { 1,2, 4 }

       BC(20,30) = { 0, 60,... }

       BC(4,6,8) = { 0, 24,... }

a)  $4\notin ƯC(12,18)$

b)   $6\in ƯC(12,18)$

c)   $2\in ƯC(4,6,8)$

d)   $4\notin ƯC(4,6,8)$

e)   $80\notin BC(20,30)$

g)    $60\in BC(20,30)$

h)   $12\notin BC(4,6,8)$

i)    $24\in BC(4,6,8)$