Review the Math Notes box in section 5.3.2

Note that there are terms missing in the table.
$a_1 = 1$ while $a_5 = −7$.
What is the common difference?

Review the Math Notes box in section 5.3.2
Is this Arithmetic or Geometric?

This is a Geometric sequence.

$\frac{a_{2}}{a_{1}}=\frac{18}{10}=1.8$

$\frac{a_{3}}{a_{2}}=\frac{32.4}{18}=1.8$

So $r = 1.8$

$a_4 = a_3 · 1.8$

$a_4 = 32.4 · 1.8$

$a_4 = 58.32$