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Re: conversion of sin to cos

• To: mathgroup at smc.vnet.net
• Subject: [mg87075] Re: conversion of sin to cos
• From: Narasimham <mathma18 at hotmail.com>
• Date: Mon, 31 Mar 2008 02:03:31 -0500 (EST)
• References: <fsnbe4$da2$1@smc.vnet.net>

On Mar 30, 11:19 am, glenn... at gmail.com wrote:
> Hello all,
>
> Here is my problem. I have a very long product of cosines with
> arguments of the form k*Pi, k being integers.
> I want to convert this expression to a sum of cosines. TrigReduce
> works fine, except that it converts cos to sin.
> So, I want all the sin (after the TrigReduce) to be converted to cos
> and the arguments be of the form p*Pi, p being rational of course.
> It has something to do with Hold or HoldAll, but I don't know the
> details. Or perhaps the TrigReduce can be overridden
> somehow.
> Can someone help?
> Thank's for your time.

Cos[J Pi] is + or - depending on whether J is respy even/odd . If
there are m odd Pis and n even Pis, then J = m  alone decides the
sign, - for odd m and + for even m. HTH