Answer
$1,3,6,10,$
$a_{10}=55$.
Work Step by Step
$a_{1}=\displaystyle \frac{1(1+1)}{2}=1$,
$a_{2}=\displaystyle \frac{2(2+1)}{2}=\frac{6}{2}=3$,
$a_{3}=\displaystyle \frac{3(3+1)}{2}=\frac{12}{2}=6$,
$a_{4}=\displaystyle \frac{4(4+1)}{2}=\frac{20}{2}=10$,
$a_{10}=\displaystyle \frac{10(10+1)}{2}=\frac{110}{2}=55$.
