Legacy Document

acos

You can use the acos function to compute the arc cosine of a real number between -1 and +1.

double_t acos (double_t x);

x

Any floating-point number in the range -1 x 1.

DESCRIPTION

The acos function returns the arc cosine of its argument x. The return value is expressed in radians in the range [0, \x86].

 such that  for -1  x  1

The cos function performs the inverse operation $(cos(y))$ .

EXCEPTIONS

When x is finite and nonzero, the result of $acos(x)$ might raise one of the following exceptions:

• inexact (for all finite, nonzero values of x other than 1)
• invalid (if |x|>1)

SPECIAL CASES

Table 10-23 shows the results when the argument to the acos function is a zero, a NaN, or an Infinity, plus other special cases for the acos function.

Special cases for the acos function

Operation	Result	Exceptions raised
$acos(x)$ for |x| > 1	NaN	Invalid
$acos(-1)$	\x86	Inexact
$acos(+1)$	+0	None
$acos(+0)$	\x86/2	Inexact
$acos(-0)$	\x86/2	Inexact
$acos(NaN)$	NaN	None[44]
$acos(+$ )	NaN	Invalid
$acos(-$ )	NaN	Invalid

EXAMPLES

z = acos(1.0);    /* z = arccos (1) = 0.0 */
z = acos(-1.0);   /* z = arccos (-1) = \x86. The inexact exception is 
                     raised. */

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[44] If the NaN is a signaling NaN, the invalid exception is raised.