You recognize them as the first 5 powers of 2 with alternating signs,

which is the same as the first 5 powers of -2, that is,

(-2)1 + (-2)2 + (-2)3 + (-2)4 + (-2)5 

You can choose any letter as an "index" or "dummy variable", I'll
choose the letter k, but you can choose any letter:



Very often when the signs alternate, a factor of (-1)k is preferred.
In that case you would write (-2)k as (-1)k2k, and write the summation as:



Edwin