On this question: How do you find the center of a circle with a pencil and a book? , I posted a comment that was a way to solve the question, just not in a mathematical way - that's why it was a comment.
It was along the lines of the famous story of how to measure the height of a building using a barometer. Another comment along those lines is still there, so I don't understand why my comment would vanish.
Open the book, maybe the answer to your question is written somewhere..!
The flag seemed to be due to the second part about hiding the circle making the problem go away, which did seem non-constructive.
It was a borderline case, and perhaps I should have left it. Repost if you wish.
How long can my post be locked pending resolution?