difference_of_squares.rb

class Squares
  attr_reader :max
  
  def initialize(max)
    @max = max
  end
  
  def square_of_sums
    (1..max).inject(:+) ** 2
  end
  
  def sum_of_squares
    (1..max).map{ |n| n ** 2 }.inject(:+)
  end
  
  def difference
    square_of_sums - sum_of_squares
  end
end