Answer: Choice A
[tex]s_1 = 2.25, \ r = 2[/tex]
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Work Shown:
[tex]s_4 = 18\text{ is the fourth term}\\\\s_5 = r*s_4 = 18r\\\\s_6 = r*s_5 = r*(18r) = 18r^2 = 72\\\\\text{So,}\\\\18r^2 = 72\\\\r^2 = \frac{72}{18}\\\\r^2 = 4\\\\r = \sqrt{4}\\\\r = 2[/tex]
Use that r value and [tex]s_4 = 18[/tex] to find the first term
[tex]s_n = s_1*(r)^{n-1}\\\\s_4 = s_1*(2)^{4-1}\\\\18 = s_1*8\\\\s_1 = \frac{18}{8}\\\\s_1 = 2.25[/tex]
You could use [tex]s_6 = 72[/tex] in place of [tex]s_4 = 18[/tex] (just make sure to use n = 6 instead of n = 4).
