SIN(3M)SIN(3M)NAME
sin, cos, tan, asin, acos, atan, atan2 - trigonometric functions and
their inverses
SYNOPSIS
#include <math.h>
double sin(x)
double x;
double cos(x)
double x;
double tan(x)
double x;
double asin(x)
double x;
double acos(x)
double x;
double atan(x)
double x;
double atan2(y,x)
double y,x;
DESCRIPTION
Sin, cos and tan return trigonometric functions of radian arguments x.
Asin returns the arc sine in the range -pi/2 to pi/2.
Acos returns the arc cosine in the range 0 to
Atan returns the arc tangent in the range -pi/2 to pi/2.
On a VAX,
atan2(y,x) := atan(y/x) if x > 0,
sign(y)∗(pi - atan(|y/x|)) if x < 0,
0 if x = y = 0, or
sign(y)∗pi/2 if x = 0 != y.
DIAGNOSTICS
On a VAX, if |x| > 1 then asin(x) and acos(x) will return reserved op‐
erands and errno will be set to EDOM.
NOTES
Atan2 defines atan2(0,0) = 0 on a VAX despite that previously
atan2(0,0) may have generated an error message. The reasons for
assigning a value to atan2(0,0) are these:
(1) Programs that test arguments to avoid computing atan2(0,0) must be
indifferent to its value. Programs that require it to be invalid
are vulnerable to diverse reactions to that invalidity on diverse
computer systems.
(2) Atan2 is used mostly to convert from rectangular (x,y) to polar
(r,theta) coordinates that must satisfy x = r∗cos theta and y =
r∗sin theta. These equations are satisfied when (x=0,y=0) is
mapped to (r=0,theta=0) on a VAX. In general, conversions to polar
coordinates should be computed thus:
r := hypot(x,y); ... := sqrt(x∗x+y∗y)
theta := atan2(y,x).
(3) The foregoing formulas need not be altered to cope in a reasonable
way with signed zeros and infinities on a machine that conforms to
IEEE 754; the versions of hypot and atan2 provided for such a
machine are designed to handle all cases. That is why atan2(±0,-0)
= ±pi, for instance. In general the formulas above are equivalent
to these:
r := sqrt(x∗x+y∗y); if r = 0 then x := copysign(1,x);
if x > 0 then theta := 2∗atan(y/(r+x))
else theta := 2∗atan((r-x)/y);
except if r is infinite then atan2 will yield an appropriate multiple
of pi/4 that would otherwise have to be obtained by taking limits.
ERROR (due to Roundoff etc.)
Let P stand for the number stored in the computer in place of pi =
3.14159 26535 89793 23846 26433 ... . Let "trig" stand for one of
"sin", "cos" or "tan". Then the expression "trig(x)" in a program
actually produces an approximation to trig(x∗pi/P), and "atrig(x)"
approximates (P/pi)∗atrig(x). The approximations are close, within
0.9 ulps for sin, cos and atan, within 2.2 ulps for tan, asin, acos and
atan2 on a VAX. Moreover, P = pi in the codes that run on a VAX.
In the codes that run on other machines, P differs from pi by a frac‐
tion of an ulp; the difference matters only if the argument x is huge,
and even then the difference is likely to be swamped by the uncertainty
in x. Besides, every trigonometric identity that does not involve pi
explicitly is satisfied equally well regardless of whether P = pi. For
instance, sin(x)**2+cos(x)**2 = 1 and sin(2x) = 2sin(x)cos(x) to within
a few ulps no matter how big x may be. Therefore the difference
between P and pi is most unlikely to affect scientific and engineering
computations.
SEE ALSOmath(3M), hypot(3M), sqrt(3M), infnan(3M)AUTHOR
Robert P. Corbett, W. Kahan, Stuart I. McDonald, Peter Tang and, for
the codes for IEEE 754, Dr. Kwok-Choi Ng.
4th Berkeley Distribution May 12, 1986 SIN(3M)