Approximating Pi with Polygons
You asked, 


  Would it make it better if I had a function called PiApproximation?


The code in main does just one thing so it would not be an improvement to introduce a new function like this — the question would then be, what's the point of main?
There is a dependency between the initial value of n and the initial values of innerPoly and outerPoly. It would make sense to put all the initialization code together.
Because n can never be negative it could be unsigned.
The loop:

for (int i=3; n&lt;=8192; n=power(i,2), i++)


is quite hard to follow because the loop variable is i but the termination condition is on a different variable n. Because the loop variable i is not used, it would be easier to follow the logic if you wrote:

for (; n &lt;= 8192; n *= 2)


(This also avoids the need for the power function.)
The l modifier has no effect on the printf format specifier %f, and so should be omitted.