I have create a shape in inkscape and now want to finish it by creating a smooth arc between the last 2 end points.

[![diagram][1]][1]

**Edit**
In response to step 9, union splits the shape not joins it. It seems to create a union of the arcs not join the shape.

Select all

[![Select all lines and arcs][2]][2]

Path -> Union

[![union split shape][3]][3]


  [1]: https://i.stack.imgur.com/IDBgy.png
  [2]: https://i.stack.imgur.com/Ka0Tf.png
  [3]: https://i.stack.imgur.com/h9Uzr.png