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Student Support Forum: 'Inconsistent ArcCos Behavior' topicStudent Support Forum > General > Archives > "Inconsistent ArcCos Behavior"

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Kevin
10/12/07 4:28pm

I am using Mathematica 6.0.0 and have run into what appears to be an inconsistency with the ArcCos function. ArcCos[1] returns 0. But ArcCos[exp], where exp evaluates to 1, returns a complex number not equal to 0. Below I copied a Mathematica notebook that illustrates my question. I have also attached the notebook to this message.

In[1]:= (* Compute an expression *)
a = 17.0677;
b = 1;
c = 18.0677;
exp = (-a^2 + b^2 + c^2)/(2 b c)

Out[4]= 1.

In[5]:= (* Verify that the expression equals 1 *)
exp == 1

Out[5]= True

In[6]:= (* ArcCos of 1 equals 0, as expected *)
ArcCos[1]

Out[6]= 0

In[7]:= (* But ArcCos of our expression doesn't equal 1 *)
ArcCos[exp]

Out[7]= 0.+ 2.10734*10^-8 \[ImaginaryI]

Attachment: ArcCos.nb, URL: ,

Subject (listing for 'Inconsistent ArcCos Behavior')
Author Date Posted
Inconsistent ArcCos Behavior Kevin 10/12/07 4:28pm
Re: Inconsistent ArcCos Behavior yehuda ben-s... 10/16/07 03:01am
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