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Re: Rewriting of Trigonometric Functions

  • To: mathgroup at
  • Subject: [mg26843] Re: Rewriting of Trigonometric Functions
  • From: "Paul Lutus" <nospam at>
  • Date: Thu, 25 Jan 2001 01:13:21 -0500 (EST)
  • References: <94m8st$>
  • Sender: owner-wri-mathgroup at

"Thomas Engelhardt" <thomas-a.engelhardt at> wrote in message
news:94m8st$3mn at
> Dear Mathgroup members,
> I have just started to use Mathematica and directly ran into a problem:
> I have rather lengthy expressions resulting from putting a sum of two
> sinusodial swings to the power of three (Two-tone Intermodulation).
> In order for me to recognise the contained frequencies I want Mathematica
> rewrite the contained Trigonometric functions in certain ways, namely
> [sin(a)]^2  should be transformed into 1/2*(1+cos2x)
> and for higher powers accordingly.
> Furthermore:
> sin(a)*sin(b) should be changed to 1/2*[cos(a-b) - cos(a+b)]
> I have tried all kinds of "Expand" and "Trig..." operations but
> always keeps the powers on the trigonometric functions and doesn't change
> the multiplications either.
> Does anybody have an idea how I can get Mathematica to transform the
> expressions in the desired way?

Why not write your own functions and use them instead of the default
functions? This is by far the simplest way to get a non-standard treatment
of the described functions.

Paul Lutus

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