calculus - Why do we use the form of f(x+delta X) = equation vs y + delta y equation?

Question

It doesn’t make sense which step to use when finding the derivative . When should I apply the other step and what makes those two different?

question from:https://stackoverflow.com/questions/65837548/why-do-we-use-the-form-of-fxdelta-x-equation-vs-y-delta-y-equation

Answer 1

Waitting for answers