I've been waiting for someone to mention the Division Operation.

A division followed by a union gets you what you want (but does leave excessive nodes on the curve):

http://www.majhost.com/gallery/ffaat/inkscape/corner.png

And if you want to work with nodes and are using 4.7 with node snapping, you can just add a node then move two nodes and perform a union:

http://www.majhost.com/gallery/ffaat/inkscape/corner2.jpg

-Rob A>