maximal and minimal in elements of hasse diagram

Question

Hi my professor says this hasse diagram has 2 minimal and 2 maximal member

But why a3 is minimal ? because a5 < a3

And why a8 is maximal because a7 > a8

Answer 1

Here $a_3$ and $a_5$ are not comparable, i.e., neither $a_3<a_5$ nor $a_5<a_3$, likewise $a_7$ and $a_8$ are not comparable.