let A, B be ext-real-membered set ; :: thesis:
assume that
A1: A is interval and
A2: B is interval ; :: thesis:
given z being ExtReal such that A3: z in A and
A4: z in B ; :: according to MEMBERED:def 20 :: thesis:
for x, y, r being ExtReal st x in A \/ B & y in A \/ B & x <= r & r <= y holds
r in A \/ B

proof

let x, y, r be ExtReal; :: thesis:
assume that
A5: x in A \/ B and
A6: y in A \/ B and
A7: x <= r and
A8: r <= y ; :: thesis:

per cases by A5, A6, XBOOLE_0:def 3;

suppose ( x in A & y in A ) ; :: thesis:

then r in A by A1, A7, A8, Th80;
hence r in A \/ B by XBOOLE_0:def 3; :: thesis:

end;

suppose that A9: x in A and
A10: y in B ; :: thesis:

per cases ;

suppose r <= z ; :: thesis:

then r in A by A1, A3, A7, A9, Th80;
hence r in A \/ B by XBOOLE_0:def 3; :: thesis:

end;

suppose z <= r ; :: thesis:

then r in B by A2, A4, A8, A10, Th80;
hence r in A \/ B by XBOOLE_0:def 3; :: thesis:

end;

end;

end;

suppose that A11: x in B and
A12: y in A ; :: thesis:

per cases ;

suppose z <= r ; :: thesis:

then r in A by A1, A3, A8, A12, Th80;
hence r in A \/ B by XBOOLE_0:def 3; :: thesis:

end;

suppose r <= z ; :: thesis:

then r in B by A2, A4, A7, A11, Th80;
hence r in A \/ B by XBOOLE_0:def 3; :: thesis:

end;

end;

end;

suppose ( x in B & y in B ) ; :: thesis:

then r in B by A2, A7, A8, Th80;
hence r in A \/ B by XBOOLE_0:def 3; :: thesis:

end;

end;

end;

hence A \/ B is interval by Th88; :: thesis: