atan2 *y*, *x*

Returns the arctangent of
*y*`/`

*x* in
the range - to .
A quick way to get an approximate value of is to say:

$pi = atan2(1,1) * 4;

For the tangent operation, you may use the `POSIX::tan()`

function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }