Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes cosine of $_ .
$_
For the inverse cosine operation, you may use the Math::Trig::acos() function, or use this relation:
Math::Trig::acos()
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
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