You have a line, a point and a circle (randomly placed). Make a line segment with:
1. Fist vertex on the line
2. Second vertex on the circle edge
3. The point is the exact middle of the circle
The result is something like the attachment (hand drawn), but you must perform an actual build of it (step by step structure, so when I move the point for example I can follow your tutorial and be able to build the segment again).
I strongly suggest you to use some online geometry system to perform your builds, because it will be much easier. For example
http://geonext.uni-bayreuth.de/index.php?id=2481&SPRACHE=en (you can directly attach the ready project in a reply). If not - make a slideshow of jpegs
P.S. It is not always possible to build this segment! We will only focus on the situation when it is possible however!
