• To: mathgroup at smc.vnet.net
• Subject: [mg71180] Re: [mg71099] Question about trig simplify
• From: Bob Hanlon <hanlonr at cox.net>
• Date: Thu, 9 Nov 2006 03:41:35 -0500 (EST)

```Use TrigReduce

2*Sin[a]*Cos[a]//TrigReduce

sin(2 a)

-2*Sin[a]*Cos[a]//TrigReduce

-sin(2 a)

Bob Hanlon

---- Robert Pigeon <robert.pigeon at videotron.ca> wrote:
>
> 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
>

```

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