Chapter 12 - Sequences; Induction; the Binomial Theorem - 12.3 Geometric Sequences; Geometric Series - 12.3 Assess Your Understanding - Page 824: 28

$2187$

We know that $a_n=a_1r^{n-1}.$ where $r$=common ration (we can get this by dividing any two consecutive terms) and $a_1$= the first term Here $a_1=1$ and $r=\frac{a_2}{a_1}=\dfrac{3}{1}=3$ Hence, here we have $a_n=1(3)^{n-1}.$ Therefore $a_{8}=1(3)^{8-1}\\=2187.$