JAMB 1995 · UME · Q10

If $S = \{x : x^2 = 9, x > 4\}$ , then $S$ is equal to

Worked solution (try it first)

$x^2 = 9$ gives $x = 3$ or $x = -3$ .
Neither is greater than 4, so no number satisfies both conditions and $S$ is the empty set, $\varnothing$ , option C.