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Re: [avr-gcc-list] error with acos() ?

From: Jamie Morken
Subject: Re: [avr-gcc-list] error with acos() ?
Date: Sat, 27 Jul 2002 18:38:59 -0700


Sorry to reply to my own email :)

I think I may have figured out the problem with acos(x);

the atan2 has the arguments backwards I think.
atan2(x,y) should be atan2(y,x)

   acos(x)=2*atan(sqrt((1-x)/(1+x)))       x>=0
          =pi - 2*atan(sqrt((1+x)/(1-x)))  x<0

   atan2(y,x)=atan(y/x)       x>0
   atan2(y,x)=atan(y/x)+pi    x<0, y>=0
   atan2(y,x)=pi/2            x=0, y>0
   atan2(y,x)=atan(y/x)-pi    x<0, y<0
   atan2(y,x)=-pi/2           x=0, y<0
   atan2(0,0) is undefined and should give an error.

Maybe the checking of x to 0 is mixed up somehow?

Jamie Morken

> Hi,
> I am using acos() and think there is an error with it.
> Here is the line I use:
> temp4 = acos(0.999911);
> This should return 0.013316 I think,
> but it returns 0.0878546
> I am using avr-gcc 3.2
> cheers,
> Jamie Morken
> avr-gcc-list at http://avr1.org

avr-gcc-list at http://avr1.org

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