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Ian Staniforth
Thank you for your pointers on circles of equal altitude.  On reflection, I’ve used the explanation myself.  I’d just forgotten.  Unfortunately, I set off in the wrong direction by misreading your post as two circles of equal radius, not altitude.  That didn’t seem to fit the problem, so I was baffled.  My main task is going to be translating the diagram into equations, unless I’ve effectively done that already with my own diagram.  As to 3D vectors, that’ll be fun.  2D is just 6^th Form Applied Maths.  For 3D, I don’t even know what the letters and squiggles mean.  However, I hope to learn from your book, which I’ve just ordered from Amazon Prime.  Dave P