# Computing the divergence of the normal vector

**URL:** <https://community.freefem.org/t/computing-the-divergence-of-the-normal-vector/68>\
**Category:** General Discussion\
**Created:** [June 5, 2019, 8:36am UTC](https://community.freefem.org/t/computing-the-divergence-of-the-normal-vector/68 "2019-06-05T08:36:56Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![sarahf](https://avatars.discourse-cdn.com/v4/letter/s/9de053/32.png) [@sarahf](https://community.freefem.org/u/sarahf)\
**Post date:** [June 5, 2019, 8:36am UTC](https://community.freefem.org/t/computing-the-divergence-of-the-normal-vector/68/1 "2019-06-05T08:36:56Z")

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Dear all,  
I want to compute the shape derivative of a form and hence i need to solve a problem with _int1d(Th)( f_ (**dx(N.x)+dy(N.y)**) )\*.  
This does not work as such, and since I am optimizing my shape, I don’t have an explicit form of my border so I cannot compute the normal by myself.  
Do you know how this could be done?  
Thanks!