From: Ada novice <ycalleecharan@gmx.com>
Subject: Arctan: to use with single or with double arguments?
Date: Fri, 24 Sep 2010 14:32:40 -0700 (PDT)
Date: 2010-09-24T14:32:40-07:00 [thread overview]
Message-ID: <908e7aea-ed77-4dce-9f5d-e6341abc1303@s19g2000vbr.googlegroups.com> (raw)
Hi,
I'm computing a Jacobian matrix (having elements as partial
derivatives) and some matrix elements have expressions like
Z = Force * Sin (A) where A is the inverse tangent of some function,
and this function has a numerator Y and a denominator X. So I have
Z_Method1 := Force * Sin(Arctan(Y, X))
using Arctan with double arguments or we get
Z_Method2 := Force * Sin(Arctan(Y / X))
using Arctan with single a argument.
Now I know that Arctan(Y, X) gives the result between -pi to pi while
Arctan(Y / X) gives the result between -pi/2 to pi/2.
For some cases of X and Y, I could see that Z_Method1 won't be equal
to Z_Method2. In fact, Z_Method1 differs from Z_Method2 in the sign in
the following 2 cases:
---------------------------------------------------------------
CASE 1: BOTH X AND Y -VE: ANSWER IS BETWEEN -PI AND -PI/2
y is numerator and x is denominator
---------------------------------------------------------------
y = -3.000
x = -1.000
Two arguments Arctan(y,x)---Answer is between -pi to pi
Angle in rad is -1.893 and Angle in degrees is -108.435
Sine is -0.949
---------------------------------------------------------------
One argument Arctan(y/x)---Answer is between -pi/2 to pi/2
Angle in rad is 1.249 and Angle in degrees is 71.565
Sine is 0.949
and in the 2nd case:
---------------------------------------------------------------
CASE 2: X = -VE AND Y +VE: ANSWER IS BETWEEN PI/2 AND PI
y is numerator and x is denominator
---------------------------------------------------------------
y = 3.000
x = -1.000
Two arguments Arctan(y,x)---Answer is between -pi to pi
Angle in rad is 1.893 and Angle in degrees is 108.435
Sine is 0.949
---------------------------------------------------------------
One argument Arctan(y/x)---Answer is between -pi/2 to pi/2
Angle in rad is -1.249 and Angle in degrees is -71.565
Sine is -0.949
Both Z_Method1 and Z_Method2 are computing Z = Force * Sin (A) as
mentioned above. Now, a Jacobian matrix is normally used in a
linearization procedure when trying to "simplify" a nonlinear system.
This Jacobian matrix can be consequently be solved for its eigenvalues
to determine the stability of the system.
In the 2 cases above I get Z = Force * Sin (A) with different signs.
Which of Z_Method1 and Z_Method2 to choose? Z_Method1 with the double
arguments in the Arctan function or Z_Method2 with the single argument
in the Arctan function?
Thanks a lot...
next reply other threads:[~2010-09-24 21:32 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-09-24 21:32 Ada novice [this message]
2010-09-24 21:55 ` Arctan: to use with single or with double arguments? Jeffrey Carter
2010-09-25 8:44 ` Ada novice
2010-09-25 17:31 ` Jeffrey Carter
2010-09-25 21:34 ` Ada novice
2010-09-26 4:02 ` jpwoodruff
2010-09-26 8:21 ` Ada novice
2010-09-26 7:31 ` Jeffrey Carter
2010-09-26 8:24 ` Ada novice
2012-02-11 0:26 ` Jack Mitchell
2012-02-11 1:11 ` Adam Beneschan
2012-02-11 8:53 ` Simon Wright
2012-02-11 10:46 ` AdaMagica
2012-02-11 15:29 ` Simon Wright
2012-02-11 16:21 ` Georg Bauhaus
2012-02-11 16:47 ` Simon Wright
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