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Re: Exact value of Cos[Pi/17]

  • To: mathgroup at
  • Subject: [mg14851] Re: Exact value of Cos[Pi/17]
  • From: "Allan Hayes" <hay at>
  • Date: Fri, 20 Nov 1998 02:16:59 -0500
  • References: <72tt9f$>
  • Sender: owner-wri-mathgroup at

Christopher Gorski wrote in message <72tt9f$isg at>...
>In The Mathematica Book, there is an example on page 936 of Mathematica
>returning the exact answer to Cos[Pi/17].  When I try it out on my NT
>machine, however, (mathematica v.3), it simply returns Cos[Pi/17].
>I've searched throughout the book, and I can't figure out how to get my
>system to return a answer as in the example given.  It will return
>simple things, Cos[Pi], for example, returns -1, but more complex
>arguments won't work.
>Christopher Gorski
>cgorski at

Christopher (using InputForm for Output cells)




1/4*Sqrt[1/2*(15 + Sqrt[17] + Sqrt[2*(17 - Sqrt[17])] +
     Sqrt[2*(34 + 6*Sqrt[17] - Sqrt[2*(17 - Sqrt[17])] +
        Sqrt[34*(17 - Sqrt[17])] -
        8*Sqrt[2*(17 + Sqrt[17])])])]


"FunctionExpand[expr] tries to expand out special and certain other
functions \
in expr, when possible reducing compound arguments to simpler ones. \
FunctionExpand[expr, assum] expands using assumptions."


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