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/* acos()
*
* Inverse circular cosine
*
*
*
* SYNOPSIS:
*
* double x, y, acos();
*
* y = acos( x );
*
*
*
* DESCRIPTION:
*
* Returns radian angle between 0 and pi whose cosine
* is x.
*
* Analytically, acos(x) = pi/2 - asin(x). However if |x| is
* near 1, there is cancellation error in subtracting asin(x)
* from pi/2. Hence if x < -0.5,
*
* acos(x) = pi - 2.0 * asin( sqrt((1+x)/2) );
*
* or if x > +0.5,
*
* acos(x) = 2.0 * asin( sqrt((1-x)/2) ).
*
*
* ACCURACY:
*
* Relative error:
* arithmetic domain # trials peak rms
* DEC -1, 1 50000 3.3e-17 8.2e-18
* IEEE -1, 1 10^6 2.2e-16 6.5e-17
*
*
* ERROR MESSAGES:
*
* message condition value returned
* asin domain |x| > 1 NAN
*/
#define __USE_BSD
#include <math.h>
double acos(double x)
{
if (x < -0.5) {
return (M_PI - 2.0 * asin( sqrt((1+x)/2) ));
}
if (x > 0.5) {
return (2.0 * asin( sqrt((1-x)/2) ));
}
return(M_PI_2 - asin(x));
}
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