Mathematica 10 evaluates Simplify[Cos[2 ArcCos[A]]] as Cos[2 ArcCos[A]], instead of the desired 2 A^2 - 1. How do I go about making this reduction?
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1 Answer
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1
Try TrigExpand
TrigExpand[Cos[2 ArcCos[A]]]
-1 + 2 A^2
answered Oct 8, 2014 at 3:10
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4$\begingroup$ Comment: situations like this, where specialized functions such as
TrigExpandorTrigToExpare needed, often pop up. They are seemingly counterexamples to one of the Mathematica core principles: "Automate as much as possible." (wolfram.com/language/principles) $\endgroup$– murrayCommented Oct 8, 2014 at 14:16