9. The ancient Greek mathematicians associated natural numbers with polygons in the following way, defining polygonal numbers:

Triangular numbers: $1,3,6,10,15 \cdots \cdots$
Square numbers: $1,4,9,16,25 \cdots \cdots$
Pentagonal numbers: $1,5,12,22,35 \cdots \cdots$
Hexagonal numbers: $1,6,15,28,45 \cdots \cdots$
$\qquad$
Then, according to the above sequence, the 6th hexagonal number is $\qquad$
$66$