Question

Find the difference between the $8^{th}$ and $6^{th}$ triangular number if the first triangular number starts with $1.$

• A. $10$
• B. $15$
• C. $36$
• D. $21$

Answer 1

The correct option is B $15$

The $n^{th}$ triangular number can be found by using the formula:

$1+2+3+...$ up to $n$ terms
$8^{th}\text{triangular number}=1+2+3+4+5+6+7+8=36$
$6^{th}\text{triangular number}=1+2+3+4+5+6=21$
$\text{Difference}=36-21=15$