I have a variable A = g / (1 + g R) Actually, if the quantity (g R) >> 1 it is possible to write A as A ≈ 1/R. How can I do this in Mathematica? I tried with Assumptions, Refine, Simplify, but i can get the simplified form.
If you rearrange the equation a bit, it is straightforward. Define a new variable x = g R. Then g = x/R and A = g/(1+g R) = (x/R)/(1+x). What you wish to calculate is the limit (as suggested by Rojo)
As b.gatessucks suggested in his comment above, this is a good spot for
which yields the result
Now you only need adjust the number of terms in the series expansion to suit your purposes...