Write down the next $3$3 triangular numbers.

$1$1, $3$3, $6$6, $10$10, $15$15, $\editable{}$, $\editable{}$, $\editable{}$

Easy

< 1min

Beginning with the eighth triangular number, write down the next $3$3 triangular numbers is this series.

Easy

< 1min

Find the $6$6th triangular number.

Medium

1min

Find the $9$9th triangular number.

Medium

< 1min

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