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Question about trig simplify

• To: mathgroup at smc.vnet.net
• Subject: [mg71099] Question about trig simplify
• From: Robert Pigeon <robert.pigeon at videotron.ca>
• 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.

But

-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
Simplify.

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

Robert Pigeon
TZ = -5 

• Follow-Ups:
  • Re: Question about trig simplify
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>