Simplifying formal expression thanks to numerical value

Hi all,

i'm new in Maxima but i think he can do what i want.

I want to simplify formal expression given that i know numerical evaluation
of some terms in the expression.
Let's take an example to clarify :

i want to symplify the expression expr :
expr: a*sqrt(b+c) ;
By numerical application, i know that c is negligible compared to b (i.e.
c/b << 1), so i'm looking to a maxima function that will give me the results
expr: a*sqrt(b)

I don't know if this function exists in maxima. I didn't find it in the
documentation.
But i'm sure there is a way to do it.
Do you have any hints ?

Thanks beforehand

best regards,

Vincent