**1. The problem statement, all variables and given/known data**

ε = xe

^{-x}

^{2}

L = L

**2. Relevant equations**

ε = ΔL/L

**3. The attempt at a solution**

xe^{-x}^{2} = (ΔL – L)/L

∴ ΔL = Lxe^{-x}^{2} + 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!

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