9. Ancient Greek mathematicians associated natural numbers with polygons in the following way, defining polygonal numbers: Triangular numbers: 1, 3, 6, 10, 15...
Square numbers: $1, 4, 9, 16, 25, \ldots$
Pentagonal numbers: $1, 5, 12, 22, 35, \ldots$
Hexagonal numbers: $1, 6, 15, 28, 45, \ldots$
$\qquad$
Then, according to the above sequence, the 8th hexagonal number is ( )