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:
It only requires a to be substituted by a+b.
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)
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?)