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.