How many interior diagonals does a regular nine-sided polygon have?  (An $\emph{interior diagonal }$ is a segment connecting two vertices which do not have an edge between them.)
Question:
How many interior diagonals does a regular nine-sided polygon have?  (An $\emph{interior diagonal }$ is a segment connecting two vertices which do not have an edge between them.)
Answer:
Let's think step by step.

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How many interior diagonals does a regular nine-sided polygon have?  (An $\emph{interior diagonal }$ is a segment connecting two vertices which do not have an edge between them.)
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