# Chapter 8, Sequences and Series - Chapter 8 Review - Exercises: 4

$a_1=1$ $a_2=3$ $a_3=6$ $a_4=10$ $a_{10}=55$

#### Work Step by Step

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

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