1. The problem statement, all variables and given/known data ε = xe-x2 L = L 2. Relevant equations ε = ΔL/L 3. The attempt at a solution xe-x2 = (ΔL - L)/L ∴ ΔL = Lxe-x2 + L I really don't understand how the solution got "dx" on the first step, also the delta L. I want to understand the first step, the rest of the solution I understand. Thanks!