Prove that
proof
A
:
'
-
u
{A
B
u
XE
.
but
,
U
F
'
.
UF
✗ C-
.
is
✗
(
u#
A
that
is a
the
but
x
✗ C-
c- U
}
a
If
x
:
as
,
only
U IF
for
x is a
of the two sets
member of
{
that
family { A
soine
some
B
}
,
requires
set
A. A C- IF →
see u
the class
{A
,
in
see
B
A)
}
(1)
containing any object
member of at least one
of the sets in #
,
then
KEB
says
at¥¥ne
u B →
defined
V
,
definition of
x is
•
REA
xE A V XE B
and the
B
u
B iff ✗ C- A V XE B
comprise
that
A
=
x:
member of
that
}
{
=
A
B
{A
,
B
A UKE B
if F-
so
=
{A B }
,
} → see A V REB
is
precisely the
entrance
'
.
.
requirement for A u B
x tu { A B } → see
(2)
Au B
,
(1)
i.
A
U{
(2)
→ act U
A, B }
=
{A
A v13
,
B
}=
a
CE AU B
>