verify arithmetic progression in ruby

arithmetic_progression.rb

class Array

	<<-DOC
		8, 3, -2, -7, -12, ...
		3 - 8 = -5
		-2 - 3 = -5
		-7 - (-2) = -5
	  -12 - (-7) = -5

		d = −5.

		FORMULA 

		Interpolation of terms in an arithmetic progression:

		Interpolate differential or arithmetic means between two numbers, it is to build an arithmetic progression having at Extreme Numbers given .
		They are the ends A and B , and the number of media interpolating a m .

		d = b-a/m+1


	DOC

	def arithmetic_progression?
		((self.last - self.first) % (self.size-1)).zero?
	end
end

puts [1, 3, 5, 7, 9, 11, 13].arithmetic_progression? #true
puts [1, 3, 5, 7, 9, 11, 12].arithmetic_progression? #false