Infinite Series Problem

[chazzs]

For my Calc III class lecture I have the following infinite series for the students:

-9 + 27/4 - 81/16 + 243/64 - 729/256 ...

This is a geometric series with the first term a = -9 and the common ratio r = -3/4.

The infinite series value is S = a/(1-r) = -9/(1-(-3/4)) = -36/7

When entered into the Prime, regardless how I enter it I get the result of +/- inf, not -36/7.

I'm entering it as the summation as k goes from 1 to infinity, of (-9)(-3/4)^(k-1). I've tried it various ways and can never get the result -36/7.

Any ideas.

C