AC= 10cm because B is the half point between A and C so your adding AB= 4.5 cm and BC= 5.5 cm you get a total of 10 cm, so your answer would be 10 cm
The wording of this question is a bit confusing... You can't write a sequence in sigma notation, but rather a series or sum. I think the question is asking you to write the sum of the sequence,

which would be

in sigma notation.
To do this, notice that the denominator in each term is a power of 2, starting with
and ending with
. So in sigma notation, this series is

When dividing by a multiple of 10, depending on if it's greater than 0, The decimal place would move. In this case, the decimal place in 34.5 would move more to the left making the answer 3.45.