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Extensive replacement of trigonometric functions

• To: mathgroup at smc.vnet.net
• Subject: [mg125007] Extensive replacement of trigonometric functions
• From: Mauro <pippo at hotmail.com>
• Date: Fri, 17 Feb 2012 06:26:41 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com

Hello to everybody.

I have this problem: I would like to replace in a long expression all 
the occurrences of:

Sin[\[Theta]_ + \[Pi]/6] and Sin[\[Theta]_ - \[Pi]/6]

with respectively:

-Cos[\[Theta] + (2 \[Pi])/3] and Cos[\[Theta] - (2 \[Pi])/3]

(which actually are the same thing).
Regretfully, the application of the rules:

Sin[\[Theta]_ + \[Pi]/6] -> -Cos[\[Theta] + (2 \[Pi])/3]
Sin[\[Theta]_ - \[Pi]/6] -> Cos[\[Theta] - (2 \[Pi])/3]

results in a flop, since sine functions stubbornly appear again!

Can you help me?

Thank you in advance

Mauro