Trig Error in CAS
02-12-2016, 06:42 PM (This post was last modified: 02-12-2016 07:06 PM by chazzs.)
Post: #1
 chazzs Junior Member Posts: 22 Joined: Feb 2015
Trig Error in CAS
Can anyone explain to me (or recreate the bug) why acos(cos(2)) = 2pi - 2, and not 2, which you get when evaluating acos(cos(2.)). Works fine on the home side.

It seems 1 is the only (integer) value that is correct: acos(cos(1))=1.

Worse is acos(cos(4)) = -2 pi + 4 = -2.2831 (Should be positive, or 2 pi - 4).

These are all in radian mode. Degree mode seems to work fine. A little troubling, or I'm missing something.

C
02-12-2016, 10:19 PM (This post was last modified: 02-12-2016 10:21 PM by DrD.)
Post: #2
 DrD Senior Member Posts: 1,133 Joined: Feb 2014
RE: Trig Error in CAS
Wolfram says: http://www.wolframalpha.com/input/?i=aco...%284%29%29

Here is my screenshot (Exact mode toggled):

Attached File(s) Thumbnail(s)

02-13-2016, 12:30 AM
Post: #3
 chazzs Junior Member Posts: 22 Joined: Feb 2015
RE: Trig Error in CAS
Interesting. Here's my screenshot.
02-13-2016, 03:29 AM (This post was last modified: 02-13-2016 09:15 PM by compsystems.)
Post: #4
 compsystems Senior Member Posts: 1,374 Joined: Dec 2013
RE: Trig Error in CAS
HOME MODE (CNES ): Computer Numeric Expert System

ACOS(COS(4)) [ENTER] 2.28318530718 ok
acos(cos(4)) [ENTER] 2.28318530718 ok

CAES MODE: Computer Algebra Expert System

flag on: Simplify Maximum
ACOS(COS(4)) [ENTER] -2*π+4 approx(Ans) −2.28318530718 ?
acos(cos(4)) [ENTER] -2*π+4 approx(Ans) −2.28318530718

SUGGESTION FOR THE FOLLOWING FIRMWARE

for CAS mode, show history with 2 outputs (Exact result & Decimal Approximation) , as does wolfram CAS

ACOS(COS(4)) [ENTER]
____________________ 2*π-4
____________________ 2.28318530718

Is a good idea?
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