Step by step
The question
Question: find A′, when ξ is the numbers 1 to 10, A is {2, 4, 6, 8} and B is {1, 2, 3, 4}.
ξ = {1, 2, …, 10}
A = {2, 4, 6, 8}
B = {1, 2, 3, 4}
1 Read the sets
A set is a list of numbers in curly brackets.
Each number is an element.
ξ is the set of everything we look at.
ξ = everything we look at
A′ = not in A
A′ means: the elements in ξ but not in A.
A′ = = in ξ but not in A
2 Draw the Venn diagram
3 Shade the part
A′ is in ξ but not in A.
On the diagram: B only, outside the circles.
4 Read the answer
Careful!
The complement is not the other set.
It is everything in ξ that is not in the set.
Careful: ′ = everything in ξ that is NOT in the set
The shaded part holds 1, 3, 5, 7, 9, 10.
We list them in order, each once.
A′ = {1, 3, 5, 7, 9, 10}
5 Check
Let's check by counting.
Everything, 10, minus the 4 inside: 6.
The same count.
So it is right!
n(ξ) − n(A) = 10 − 4 = 6
Let's look back.
First: read the sets.
Then: draw the Venn diagram.
Then: shade the part.
Then: read the answer.
Last: check.
Answer
A′ = {1, 3, 5, 7, 9, 10}