We obtain a formula after some mathematical manipulations as: *y=5+log(sin(a))*.

Now I want to explain how we can obtain another formula based on what we already derived. Compare:

1. > 
> It only requires *a* to be substituted by *a+b*. 
> 
> 
>
2. > 
> We only need to set *a* to *a+b*.
> 
> 
>

I think the first sentence is focused on replacement; It says we can reach to a certain formula through replacing something with something else. (I prefer this sentence)

However, the second sentence implying a variable that can take multiple values and we set the variable to the of interest value, *a+b*. (I don't prefer this sentence)

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Questions:

Q1: Is it correct to use passive form of the infinitive as what I've used in #1?

Q2: Is there any situation in which we must choose only one of them?

Q3: How can I explain such processes correctly? (Are there better sentences?)

Thanks.