Answer
$23.40
Work Step by Step
First, we find that the common difference is 50. This allows us to find the next terms in the sequence, given the starting number. Note, to get a given term, subtract the common difference from the previous term.
We are asked to find the 9th term. Instead of using the method above, we will use:
$(9-1)(-.1)+3=2.2$
Since this sequence is geometric, we can use:
$S_n=n/2(a_1+a_n)=9/2(3+2.2)=23.4$