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(libc.info)Inverse Trig Functions

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Inverse Trigonometric Functions
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   These are the usual arc sine, arc cosine and arc tangent functions,
which are the inverses of the sine, cosine and tangent functions
respectively.

 - Function: double asin (double X)
 - Function: float asinf (float X)
 - Function: long double asinl (long double X)
     These functions compute the arc sine of X--that is, the value whose
     sine is X.  The value is in units of radians.  Mathematically,
     there are infinitely many such values; the one actually returned
     is the one between `-pi/2' and `pi/2' (inclusive).

     The arc sine function is defined mathematically only over the
     domain `-1' to `1'.  If X is outside the domain, `asin' signals a
     domain error.

 - Function: double acos (double X)
 - Function: float acosf (float X)
 - Function: long double acosl (long double X)
     These functions compute the arc cosine of X--that is, the value
     whose cosine is X.  The value is in units of radians.
     Mathematically, there are infinitely many such values; the one
     actually returned is the one between `0' and `pi' (inclusive).

     The arc cosine function is defined mathematically only over the
     domain `-1' to `1'.  If X is outside the domain, `acos' signals a
     domain error.

 - Function: double atan (double X)
 - Function: float atanf (float X)
 - Function: long double atanl (long double X)
     These functions compute the arc tangent of X--that is, the value
     whose tangent is X.  The value is in units of radians.
     Mathematically, there are infinitely many such values; the one
     actually returned is the one between `-pi/2' and `pi/2'
     (inclusive).

 - Function: double atan2 (double Y, double X)
 - Function: float atan2f (float Y, float X)
 - Function: long double atan2l (long double Y, long double X)
     This function computes the arc tangent of Y/X, but the signs of
     both arguments are used to determine the quadrant of the result,
     and X is permitted to be zero.  The return value is given in
     radians and is in the range `-pi' to `pi', inclusive.

     If X and Y are coordinates of a point in the plane, `atan2'
     returns the signed angle between the line from the origin to that
     point and the x-axis.  Thus, `atan2' is useful for converting
     Cartesian coordinates to polar coordinates.  (To compute the
     radial coordinate, use `hypot'; see Note: Exponents and
     Logarithms.)

     If both X and Y are zero, `atan2' returns zero.

   ISO C99 defines complex versions of the inverse trig functions.

 - Function: complex double casin (complex double Z)
 - Function: complex float casinf (complex float Z)
 - Function: complex long double casinl (complex long double Z)
     These functions compute the complex arc sine of Z--that is, the
     value whose sine is Z.  The value returned is in radians.

     Unlike the real-valued functions, `casin' is defined for all
     values of Z.

 - Function: complex double cacos (complex double Z)
 - Function: complex float cacosf (complex float Z)
 - Function: complex long double cacosl (complex long double Z)
     These functions compute the complex arc cosine of Z--that is, the
     value whose cosine is Z.  The value returned is in radians.

     Unlike the real-valued functions, `cacos' is defined for all
     values of Z.

 - Function: complex double catan (complex double Z)
 - Function: complex float catanf (complex float Z)
 - Function: complex long double catanl (complex long double Z)
     These functions compute the complex arc tangent of Z--that is, the
     value whose tangent is Z.  The value is in units of radians.