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Set Theory
hard
0:00.0
What is
A
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t
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A
A \\Delta A
A
D
e
l
t
a
A
?
A
A
A
A
B
e
m
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t
y
s
e
t
\\emptyset
e
m
pt
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t
C
U
U
U
D
A
′
A'
A
′
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