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If $a$, $b$ and $d$ are the lengths of a side, a shortest diagonal and a longest diagonal, respectively, of a regular nonagon (9-sided polygon). Find an expression that relates $a$, $b$, and $d$.
If $a$, $b$ and $d$ are the lengths of a side, a shortest diagonal and a longest diagonal, respectively, of a regular nonagon (9-sided polygon). Find an expression that relates $a$, $b$, and $d$.