Variable replacement in only parts of an expression

Questions
1

Hi!

Suppose I have an expression like A = ( Log[s+t] + s^2 + t ~s + u ). I want to substitute s by a and t by b ONLY inside Log[s+t] without affecting the remaining part. Is there any way to do this? If we directly substitute s by a and t by b then Mathematica will do the substitution in every places but I want to substitute only in some selected places namely only in the arguments of a function. How can I do that? Hope the problem is clear.

Thank you.

2

.

In[1]:= A = (Log[s + t] + s2 + t s + u);A /. Log[s + t] -> Log[a + b]

Out[2]= s2 + s t + u + Log[a + b]
3

Or

In[2]:= (Log[s + t] /. {s -> a, t -> b}) + s2 + t s + u

Out[2]= s2 + s t + u + Log[a + b]