Related knowledge:

The relationship between (1) elements and sets is defined by the following formula.

Belong to

express

(Note that it cannot be represented by inclusion)

1 is contained in {1, 2}, which is incorrect.

(2) The relationship between set and set.

Tolerance or true tolerance

formulate

(It can't be represented by belonging, but it can be equal)

{1, 2} ∈ {1, 2,3} This is not right.