130
Belonging to a set
Textbook: Part 4, lesson 130, pp. 63–65
GoalAn element belonging to a set: the signs ∈ and ∉.
New words
belongs to · tegishlidoes not belong to · tegishli emasthe sign ∈ · belgi ∈two-digit number · ikki xonali son
Explanation
If an element is in a set, it belongs to the set and we write ∈. If an element is not in the set, it does not belong and we write ∉. For example, if A = {2, 4, 6}, then 4 ∈ A and 5 ∉ A. If an expression is given, we first calculate its value and then decide whether it belongs to the set.
Worked examples
Let A = {1, 3, 5, 7, 9}. The number 7 is in A, so 7 ∈ A. The number 8 is not in A, so 8 ∉ A.
M is the set of two-digit numbers. 45 ∈ M because 45 has two digits. 6 ∉ M (one digit) and 120 ∉ M (three digits).
Class activity
“Inside or outside the circle”: a big circle is drawn on the floor with chalk. The teacher names a set (for example “multiples of 5”) and calls out a number. The child steps into the circle and says “∈”, or stays outside and says “∉”.
Practice
1
T is the set of two-digit multiples of 9. Put the sign: 36 … T.
∈
2
Q is the set of even numbers less than 100. Put the sign: 57 … Q.
∉
3
A = {a, b, d, e}. Put the sign: e … A.
∈
4
K is the set of three-digit numbers. Put the sign: 25 × 4 … K.
∈
5
K is the set of three-digit numbers. Put the sign: 200 : 4 … K.
∉