How can I get Mathematica to simplify the following expression?


I'd like the answer to be ArcCos[1/Sqrt[3]], which seems manifestly "simpler" to me. If I use Simplify, it leaves it as it is. A related question is: howcome


does not return True or False, but rather returns back the same expression?

  • 2
    $\begingroup$ FullSimplify[Pi - ArcCos[-1/Sqrt[3]] - ArcCos[1/Sqrt[3]]] performs 0. $\endgroup$
    – user64494
    Mar 17, 2021 at 18:33

2 Answers 2


expr = Pi - ArcCos[-1/Sqrt[3]];

The simplification is available in MathematicalFunctionData

argSimp = (MathematicalFunctionData["ArcCos", 
       "ArgumentSimplifications"][[1]][z] // Activate) /. Equal :> Rule

(* ArcCos[-z] -> π - ArcCos[z] *)

expr /. (argSimp /. z :> 1/Sqrt[3])

(* ArcCos[1/Sqrt[3]] *)

EDIT: Use ComplexExpand to test the equality

Pi - ArcCos[-1/Sqrt[3]] == ArcCos[1/Sqrt[3]] // ComplexExpand

(* True *)

I would claim that your expected form is not the simplest one. I would instead propose $$ \tan ^{-1}\left(\sqrt{2}\right) $$ and do the simplification as follows

e = Pi - ArcCos[-1/Sqrt[3]];
es = ArcTan[Tan[e]]
(* ArcTan[Sqrt[2]] *)
(* True *)

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