In Inkscape I first typed text, created a circle, then made two lines from the center.
It looks like this now: 
Now, I want it to look like this with the piece cut off, which I made before:
I can't make it work. Every path boolean operation is not working. How do I cut circle like a pie?

For some reason it doesn't work if they cross just a little.
Then you select both of the triangle and circle then do the Path>Division and you'll get this:
Of course, triangle is one path with three nodes, not two lines.