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Complement of set A means
all elements of Universal set which are not in A
It is denoted by A’
Let A = {1, 2, 3, 4} , B = {3, 4, 5, 6}, C = {6, 7, 8}
and Universal set = U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
A’ = U – A
A’ = { 1, 2, 3, 4 , 5, 6, 7, 8, 9, 10} – { 1, 2, 3, 4 }
A’ = {5, 6, 7, 8, 9, 10}
Properties of Complement

Complement laws:
(i) A ∪ A′= U (ii) A ∩ A′= ∅ 
De Morgan’s law
:
(i) (A ∪ B)´ = A′ ∩ B′ (ii) (A ∩ B)′= A′ ∪ B′ 
Law of double complementation :
(A′)′= A 
Laws of empty set and universal set
∅′ = U and U′= ∅