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/Perl 5.26

cos

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] ) }

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https://perldoc.perl.org/5.26.0/functions/cos.html