(−∞,2]∪[6,∞)(-\infty, 2] \cup[6, \infty)(−∞,2]∪[6,∞)
[2,6][2,6][2,6]
(−∞,2)∪(6,∞)(-\infty, 2) \cup(6, \infty)(−∞,2)∪(6,∞)
(2,6)(2,6)(2,6)