
\draw []
   ($(A1)!\t!(Ak)$)
   --
   ($(A1)!\tt!(Ak)$)
   node [left]{$r$};

\filldraw  (P) circle(\r pt) node [anchor=south west] {$P$};

\filldraw  (A1) circle(\r pt) node [anchor=north east] {$A_1~$};

\filldraw  (A2) circle(\r pt) node [anchor=north] {$A_2~~~$};

\filldraw  (Ak) circle(\r pt) node [below] {$A_k$};

