Marco, thank you for the clarification. Your approach does work.
What I did, was clone the Red triangle.
Ctrl+Rotate this around the circle's centre so the short side is vertical.
Then, in the Shape tool, Ctrl+Drag the node from the bottom crossing on the circle to the top crossing.
Finally, rotate back into position.
This handles any given angle, or even when none is given, without using Page Options > Document Editing > Angle constraint.
Fuller instructions enclosed: SSA Construction.xar
Acorn
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