9. The ancient Greek mathematicians associated natural numbers with polygons in the following way,
Triangular numbers: $1,3,6,10,15$, $\qquad$
Square numbers: $1,4,9,16,25$, $\qquad$
Pentagonal numbers: $1,5,12,22,35$, $\qquad$
Hexagonal numbers: $1,6,15,28,45$, $\qquad$
According to the above sequence, the 10th triangular number is $\qquad$ .
$55$