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I have a Mathematica program that outputs a piecewise function, and some parts of the expression are not being recognised as specific numbers when using FullSimplify. For example, in the output there is

ArcCos[3/5] + 2 ArcSec[Sqrt[5]]

which is actually equal to $\pi$, and there is also

ArcCos[3/5] - 2 ArcCos[2/Sqrt[5]]

which is actually equal to $0$.

I was wondering if there was a way to have Mathematica recognise these sort of outputs in their more simplified form. I know that I can use numerical approximation, but was wondering if it was possible to do so while keeping the output in exact/closed form.

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@Kuba Perhaps I worded my question poorly; I am saying that the two expressions are equal to $\pi$ and $0$ respectively, and would like the output to have $\pi$ and $0$ as opposed to the trigonometric expressions above. –  Ross Pure Apr 16 at 7:57
    
@Kuba $\cos^{-1}(x)$ is not $1/\cos(x)$: N[ArcCos[3/5] - 2 ArcCos[2/Sqrt[5]]] (how do I format code in answers/comments?) –  Ross Pure Apr 16 at 8:03
    
@Kuba I know it does, that's why I used it in my question. –  Ross Pure Apr 16 at 8:05
    
@kuba Sorry read your comment wrong, just thought it was standard notation. –  Ross Pure Apr 16 at 8:06
1  
Closely related: mathematica.stackexchange.com/q/40080/121 –  Mr.Wizard Apr 16 at 9:12

2 Answers 2

up vote 3 down vote accepted

It seems that Mathematica can handle this when you switch to Exp form of expressions:

ArcCos[3/5] + 2 ArcSec[Sqrt[5]] // TrigToExp // FullSimplify
ArcCos[3/5] - 2 ArcCos[2/Sqrt[5]] // TrigToExp // FullSimplify
π
0
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Accepting this answer because it looks like it will solve any potential problem of this type. However, the two examples given are the only ones I know of, so I can't test further. Thanks. –  Ross Pure Apr 16 at 8:17

I don't think this will work in every possible case but for your examples you could write a function doing :

expr1 = ArcCos[3/5] + 2 ArcSec[Sqrt[5]];
expr2 = ArcCos[3/5] - 2 ArcCos[2/Sqrt[5]];

Solve[{Sin[x] == (Sin[expr1] // TrigExpand), 
       Cos[x] == (Cos[expr1] // TrigExpand), -π < x <= π}, x, Reals]
(* {{x -> π}} *)

Solve[{Sin[x] == (Sin[expr2] // TrigExpand), 
       Cos[x] == (Cos[expr2] // TrigExpand), -π < x <= π}, x, Reals]
(* {{x -> 0}} *)
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