..Hello

it is correct to write:

or i need write ?

Mr F says: This is the one you want.

why ? Mr F says: Let me re-write your two choices and then you tell me which onedoesn'tmake sense:

The question is :

if I need a substitution of f(x) in a formula :

I can directly replace f(x)

otherwise :

don't seem me elegant imho..

Thanks for the possible answers !