We are told to find the sum of the following infinite geometric series:

2+1/2-1/4+1/8-1/16...

I went to my professor earlier today and saw how he manipulated the series. (He replaced 2 with 3-1) Unfortunately, I had a class right after I met with him so afterwards I forgot what he showed me. He told me to try to manipulate it in a way so that you see a series that looks like:

1+r+r^2+r^3+r^4...

For the life of me I can't seem to find the spot where we left off together.