When ever you combine different elements the chemical properties change because elements are different , unless you combine the exact same elements.
Front yard
65+65+45+45=220
Back yard
45+45+35+35=160
220+160=380
380 feet
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

The language of oh yea doesn't exist :)