Write down the next $3$3 triangular numbers.
$1$1, $3$3, $6$6, $10$10, $15$15, $\editable{}$, $\editable{}$, $\editable{}$
Beginning with the eighth triangular number, write down the next $3$3 triangular numbers is this series.
Find the $6$6th triangular number.
Find the $9$9th triangular number.