Chapter 11 - Sequences; Induction; the Binomial Theorem - Section 11.3 Geometric Sequences; Geometric Series - 11.3 Assess Your Understanding - Page 844: 21

The $n^{th}$ term of the sequence is given by the formula: $a_n =5 \ (-1)^{n-1}$ and $a_5=5$

We are given: $a_{1}= 5 ; \ r=-1$ The $n^{th}$ term of the sequence is given by the formula: $a_n=a_1r^{n-1}$ $a_n =5 \ (-1)^{n-1}$ Now, the 5th term can be computed by substituting $5$ for $n$: $a_5=5 \ (-1)^{5-1} \\ = 5 \ (-1)^4 \\=5$