libc.a reference
tan
Syntax
 #include <math.h>
double tan(double x);

Description
This function computes the tangent of x (which should be given in
radians).
Return Value
The tangent of x. If the absolute value of x is finite but
greater than or equal to 2^63, the return value is 0 (since for
arguments that large each bit of the mantissa is more than Pi
).
If the value of x is infinite or NaN
, the return value is
NaN
and errno
is set to EDOM
.
Portability
ANSI/ISO C 
C89; C99

POSIX 
1003.21992; 1003.12001
