Question 872011
<pre>
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)<sup>1</sup> + (-2)<sup>2</sup> + (-2)<sup>3</sup> + (-2)<sup>4</sup> + (-2)<sup>5</sup> 

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

{{{sum((-2)^k,k=1,matrix(1,3,"","",5))}}}

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

{{{sum((-1)^k*2^k,k=1,matrix(1,3,"","",5))}}}

Edwin</pre>