June 8, 2016 - There seems to be a problem in lesson 11 (Feeling included for Ruby Object-Oriented Programming II)
The exercise outlines to add "include Math" in line two to use method cos without scope resolution operator. This however, will raise an issue in the interpreter box which stated something like that method "cos" cannot be resolved.
As a work-around I added a module Math prior to the class Angle which includes it. My own module Math contains a dummy method cos which takes radius as parameter.
module Math def cos(radius); end end class Angle include Math attr_accessor :radians def initialize(radians) @radians = radians end def cosine cos(@radians) end end acute = Angle.new(1) acute.cosine