Answer
$a_1=1$
$a_2=3$
$a_3=6$
$a_4=10$
$a_{10}=55$
Work Step by Step
We are given:
$a_{n}= \frac{n(n+1)}{2}$
We evaluate:
$a_1= \frac{1(1+1)}{2}=\frac{1*2}{2}=1$
$a_2= \frac{2(2+1)}{2}=\frac{2*3}{2}=3$
$a_3= \frac{3(3+1)}{2}=\frac{3*4}{2}=6$
$a_4= \frac{4(4+1)}{2}=\frac{4*5}{2}=10$
$a_{10}= \frac{10(10+1)}{2}=\frac{10*11}{2}=55$
