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MathGroup Archive 2006

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Question about trig simplify

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  • Subject: [mg71099] Question about trig simplify
  • From: Robert Pigeon <robert.pigeon at>
  • Date: Wed, 8 Nov 2006 06:15:25 -0500 (EST)

Hello all,
	Is this a bug?

2*Sin[a]*Cos[a] // Simplify  gives Sin[2 a] as expected.


-2*Sin[a]*Cos[a] // Simplify  gives -2*Cos[a]*Sin[a]  ...... Why?
If I do -2*(Sin[a]*Cos[a]) // Simplify I get the same answer.

This comes from the rotation matrix: r = {{Cos[a],Sin[a]},{-Sin[a],Cos[a]}}.
Then I do: r.r // Simplify. That gives: {{Cos[2 a],Sin[2 a]},{-2 Cos[a]
Sin[a],Cos[2 a]}}. It does not matter if I do a FullSimplify instead of

After if I do r.r.r // Simplify. The answer is simplified correctly.

I am using Mathematica 5.2 on Windows XP Home.

Any idea?


Robert Pigeon
TZ = -5 

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