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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)
This function computes 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).
asin fails, and sets errno to EDOM, if x is
out of range. The arc sine function is defined mathematically only
over the domain -1 to 1.
Function: double acos (double x)
This function computes 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).
acos fails, and sets errno to EDOM, if x is
out of range. The arc cosine function is defined mathematically only
over the domain -1 to 1.
Function: double atan (double x)
This function computes 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)
This is the two argument arc tangent function. It is similar to computing
the arc tangent of y/x, except that 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 section Exponentiation and Logarithms.)
The function atan2 sets errno to EDOM if both
x and y are zero; the return value is not defined in this
case.
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