# Help in proof of chain rule

1. Aug 10, 2011

### mahmoud2011

From james stewart calculus Early Transcendentals.Before he states the proof he intoduced a property of differentiable funcion
My problem is how we defined $\epsilon$ to be 0 when $\Delta x$=0

where this is not in the Domain.

2. Aug 10, 2011

### Nanas

what is the problem???

Like a piece wise Functions