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 Exponents and Logarithms.)

If both x and y are zero, atan2 returns zero.

ISO C99 defines complex versions of the inverse trig functions.