How to use Replace correctly?

Questions
1

Hello,

I’m new using Mathematica. I’m trying to use Mathematica to follow a demostration where I have to perform transformations in expressions.
I arrive to the following problem: in a part of an expression, I need to use the following replacement: Sin(x) = -Sin(-x). So I apply it as follows:

F1 = 1 + Sin[a - b]
Replace[F1, Sin[x_] -> -Sin[-x]]

I was expecting the following result: F1=1-Sin[b-a]
However, I find Mathematica gives the initial result, that is: F1=1 + Sin[a - b]
I guess Mathematica tries to simplify the expression and arrives to the initial expression.How can I avoid this? I have tried several ways, but without success…

Thank you.

2

You can use Hold to prevent evaluation of an expression:

Hold[1 + Sin[a - b]] /. Sin[x_] :> -Sin[-x]

Hold[1 - Sin[-(a - b)]]