Answer:
?=52 or ?=-52
Step-by-step explanation:
Find the geometric mean:
?=sqrt(13*208)
?=sqrt(2704)
?=±52
So ? can be either 52 OR -52. Keep in mind that since we have an odd number of terms in the sequence, -52 is a possible missing term.
13*4=52, 52*4=208
13*-4=-52, -52*-4=208
