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Re: conversion of sin to cos
*To*: mathgroup at smc.vnet.net
*Subject*: [mg87092] Re: conversion of sin to cos
*From*: Szabolcs Horvát <szhorvat at gmail.com>
*Date*: Mon, 31 Mar 2008 02:06:53 -0500 (EST)
*Organization*: University of Bergen
*References*: <fsnbe4$da2$1@smc.vnet.net>
glenn077 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.
Cos[k*Pi] == (-1)^k, so your expression is probably a bit more
complicated than that :-)
> 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.
Have you tried
expression /. Sin[a_] :> Cos[a - Pi/2]
or
expression /. Sin[a_] :> Cos[Collect[a - Pi/2, Pi]]
or
expression /. Sin[a_] :> Cos[Collect[a - Pi/2, Pi, Together]]
? It would be easier to answer if you posted a short sample of a
Sin-expression that you need converted to a Cos-expression.
> It has something to do with Hold or HoldAll, but I don't know the
> details.
Hold[] just prevents evaluation of its arguments. HoldAll is not a
function, but an attribute. It is the attribute that gives Hold[] its
magic.
> Or perhaps the TrigReduce can be overridden
> somehow.
> Can someone help?
> Thank's for your time.
>
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