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Re: TrigExpand - Odd behavior

  • To: mathgroup at
  • Subject: [mg55433] Re: TrigExpand - Odd behavior
  • From: DrBob <drbob at>
  • Date: Wed, 23 Mar 2005 05:34:43 -0500 (EST)
  • References: <>
  • Reply-to: drbob at
  • Sender: owner-wri-mathgroup at

It's a rather useless result, I agree, but completely accurate (AFAIK).

"Degree" is a symbol, so TrigExpand gave you the full expansion (using sum-angle formulas) of the linear expression 45 Degree + x. Try entering Cos[x + 45 y] // TrigExpand (with x and y undefined) and you'll see a similar expression. After the expansion, there were no rules available to simplify its terms.

This goes very differently:

TrigExpand[Cos[x + Pi/4]]
Cos[x]/Sqrt[2] - Sin[x]/Sqrt[2]

45 Degree is Pi/4, but TrigExpand didn't "think" to apply that rule before the expansion. Often mathematics comes down to WHICH rules are used in what ORDER, and TrigExpand is biased toward sum-angle formulas and such.


On Tue, 22 Mar 2005 03:51:07 -0500 (EST), Bob Stagat <stagat at> wrote:

> Try executing the following three lines and explain the last result...
> Cos[45 Degree]
> Cos[x+45 Degree]
> Cos[x+45 Degree]//TrigExpand
> Thanks...
> ~ BS ~

DrBob at

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