\[\begin{array}{*{35}{l}}

4\text{ }ta{{n}^{-1}}~1/5\text{ }\text{ }ta{{n}^{-1}}~1/239  \\

=\text{ }2\text{ }(ta{{n}^{-1}}~1/5)\text{ }\text{ }ta{{n}^{-1}}~1/239  \\

\end{array}\]

\[=\text{ }2\text{ }ta{{n}^{-1}}~5/12\text{ }\text{ }ta{{n}^{-1}}~1/239\]

Thus,

\[4\text{ }ta{{n}^{-1}}~1/5\text{ }\text{ }ta{{n}^{-1}}~1/239\text{ }=\text{ }\pi /4\]