Hello, if you use the version 2 of Librecad, here you have the solution, for previous versions it should be something similar:
let's suppose you have the picturedown here
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first you need to select modify>divide
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below the drawing area, the command line asks the entity to divide (I cutted and pasted to show)
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click on the circle, now notice that the circle has changed color and the command line asks a poit to divide
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click point A using snap bar
now click again modify>divide, click on the circle and click on point B
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and of course the final result (I clicked on the cutted part to show to you)
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That's all friend.
Bye!
Claudio
...the problem is not the problem, the problem is your aptitude about the problem...