Solved
Cone, plane, ellipse
I know a and b of an ellipse. What is the value alpha of a circular cone?

I know a and b of an ellipse. What is the value alpha of a circular cone?

Yes,

here is example when a=15, b=8 and alpha=30 degree...

..and when alpha changes to 40 degree
Observe that auxiliary view is normal to cutting plane and does not show real cone angle.
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