RE: Question about trig simplify

*To*: mathgroup at smc.vnet.net*Subject*: [mg71157] RE: [mg71099] Question about trig simplify*From*: "David Park" <djmp at earthlink.net>*Date*: Thu, 9 Nov 2006 03:39:22 -0500 (EST)

Robert, It seems strange. Also notice the following. Sin[a]*Cos[a] // Simplify Cos[a] Sin[a] My idea attitude toward Simplify is that it is great when it works the way you want, and it often does, but when it doesn't it is better to abandon Simplify and work with more specific routines, or apply Simplify or other routines to specific parts of an expression. (You can do that with rules, or with Tensorial you can often do it with MapLevelParts or MapLevelPatterns, or you can do it with ReplacePart) For your specific case how about... -2*Sin[a]*Cos[a] // TrigReduce -Sin[2 a] David Park djmp at earthlink.net http://home.earthlink.net/~djmp/ TZ != -5 From: Robert Pigeon [mailto:robert.pigeon at videotron.ca] To: mathgroup at smc.vnet.net 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