I just wonder how can I do (step by step) partially differentiating with where

Noted:

- The displacement of node u is depicted as in the attached picture.
- denotes the vector in the local coordinate of u that starts from u and ens at the tip of the arc e
- is a constant value.
- E(u) is a set of arc e which is attached to node u

Actually, I already have the answer but I want to know step by step in solving this equation. Please help. Thank you in advance.

The answer is