## Thinking Mathematically (6th Edition)

$\{\emptyset\} \in \{\emptyset, \{\emptyset\}\}$ is true
$\{\emptyset\} \in \{\emptyset, \{\emptyset\}\}$ This is correct. Let's say $A=\{\emptyset\}$ and then substitute: $A \in \{\emptyset, A\}$ This makes it clear, as $A$ is an element of a set that contains $A$.