Question 1082981
<pre>
{{{a[n]= (1/2)^n}}}

That formula will never give a negative term no matter what 
positive whole number we substitute for n.  D is the only 
choice that has only positive terms, so D is the only possible 
correct choice. 

But let's suppose we didn't notice that:

We'd plug in 1 for n

{{{a[1]= (1/2)^1}}}
{{{a[1]= 1/2}}}    <-- that rules out B, the others have 1st term 1/2

We'd plug in 2 for n

{{{a[2]= (1/2)^2}}}
{{{a[2]= 1/4}}}    <-- that rules out A, for C & D have 2nd term 1/4

We'd plug in 3 for n

{{{a[3]= (1/2)^3}}}
{{{a[3]= 1/8}}}    <-- that rules out C.

So the answer must be D, but let's check to make sure:

We plug in 4 for n

{{{a[4]= (1/2)^4}}}
{{{a[4]= 1/16}}}    <-- yes, D is the correct choice.

Edwin</pre>