# Simplifying a differential equation

1. Apr 16, 2014

### mash17

When I multiply out the first line I end up with an extra (dA/dx)*(dσ/dx). Can someone please show me how i get from the first line to the second. Thanks

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2. Apr 17, 2014

### Bill_K

The extra term is dA/dx · dσ/dx · dx · dx, which can be dropped because dx is an infinitesimal quantity and this term is second order in dx, infinitesimally smaller than the term you keep, which is first order.