1- 1 Let the set A ={ 1, {2}, a, 4, 3}, and the following proposition is true [B].

A . 2∈A； b . 1∈A； c . 5∈A； {2} Answer.

If 1-2 A and b are arbitrary sets, then their * * * isomorphic subset is [D].

A.a； B . Bc . A∪B； d？ .

1-3 let S = {N, z, q, R}, and judge whether the following proposition holds?

(1) N is included in q, Q ∈S, then n is included in (or belongs to) s [No].

(2)- 1 ∈Z, Z ∈S, then-1∈S[ No]

1-4 Let the sets a = {3 3,4} and B = {4,4} ∩? ，C = {4，3} ∩{？ }，D ={ 3，4，？ },

E = {x│x ∈R and x2-7x+ 12 = 0}, F = {4,? ,3,3},

Which two groups can be represented by equal signs? A=E，F=D，B=C

1-5 The following sets are represented by column element method.

(1)A = {x│x ∈N and x2 ≤ 9} = {1, 2,3}

(2)A = {x│x ∈N and 3-x < 3} = {1, 2}