How to substitute this math expression?

[tyanata]

May be my question is idiotic, but can you advise me how to substitute this expression
1/(1+A*X+B+C*((X)^2)). Is there some solution simailar as 1/(1+A*X)≈1*(1-A*X).
If you now some thing like this. Please help!!!!

 

[mayyan]

Re: Substitution help

Make B and X small and then u can neglect B and CX^2

 

[Hin1219]

Substitution help

What do you mean? Whats the full question?