Question 871373
<pre>
-2+4-8+16-32

Those are the powers of 2 with alternating signs,

So they are the powers of (-2).  So, like your example:

 1,2,3,4,5 is i

-2+4-8+16-32

(-2)<sup>1</sup> =  -2
(-2)<sup>2</sup> =   4
(-2)<sup>3</sup> =  -8
(-2)<sup>4</sup> =  16
(-2)<sup>5</sup> = -32

Rule for the nth term is 

nth term = (-2)<sup>n</sup>

Edwin</pre>