You are viewing the version of this documentation from Perl blead. This is the main development branch of Perl. (git commit 064a75fb477ad99293cbd4bb73afb966a3a0bc1b)
cos EXPR
cos

Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes the cosine of $_.

For the inverse cosine operation, you may use the Math::Trig::acos function, or use this relation:

sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }