=over

=item cos EXPR
X<cos> X<cosine> X<acos> X<arccosine>

=item cos

Returns the cosine of EXPR (expressed in radians).  If EXPR is omitted,
takes the cosine of L<C<$_>|perlvar/$_>.

For the inverse cosine operation, you may use the
L<C<Math::Trig::acos>|Math::Trig> function, or use this relation:

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

=back