What is the series written in sigma notation?

Correct option: D

Step-by-step explanation:

First we need to find the rate of the series, and we can do that subtracting one term by the term before:

Second term minus first: 13/10 - 1/2 = 13/10 - 5/10 = 8/10 = 4/5

Third term minus second: 21/10 - 13/10 = 8/10 = 4/5

So the series increases by 4/5 for every term, and the first term is 1/2

We also need to find the number of terms of this series.

The last term is 93/10, so we have that:

1/2 + (4/5)*k = 93/10

(4/5)k = 88/10

k = 11

The index of the summatory should go from 0 to 11.

So the correct option is D