Which of the following is the empty set?

{x / x is a real number and x2 – 1 = 0} {x / x is a real number and x2 + 1 = 0} {x / x is a real number and x2 – 9 = 0} {x / x is a real number and x2 = x + 2}

{x / x is a real number and x2 + 1 = 0}

Since x2 + 1 = 0 ⇒ x = ± i. Hence (2) is the correct answer.