# Questions tagged [geometry]

For questions about geometric shapes, congruences, similarities, transformations, as well as the properties of classes of figures, points, lines, and angles.

31,907 questions
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### how to find if it is possible to form a convex polygon with given n sides length? [duplicate]

I have given n lengths. I have to decide whether it is possible to construct an n-sided polygon with those n lengths. What are the necessary conditions or formula to check it?
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### Numberphile's “plastic number” video, question regarding “calipers” used

Sorry for the poor title, but this question is awkward. Numberphile has a new video about the plastic number, and it demonstrates it with a set of calipers with 4 prongs. https://youtu.be/...
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### Equilateral triangle on a concentric circle

Is my idea correct? 3 concentric circles of radius 1, 2 and 3 are given. An equilateral triangle is formed having its vertices lie on the side of the three concentric circles. What is the length of ...
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### What's the size of the angle? [on hold]

Suppose we have a square ABCD. Point E is placed in the square so that DEC is an equilateral triangle. On the line BC, there is the point F, while $|EB| = |EF|$. What is the size of the $\angle CEF$?