Method to return the nth value of the Fibonacci sequence

fibonacci.rb

# PHI - Some greek number
PHI   =  Rational(610,377).to_f

# Negative inverse of PHI
I_PHI = -Rational(1, PHI).to_f

# Square root of 5
SQRT_5 = 5 ** 0.5

# Fibonacci value for given number x
#
# x - An Integer representing the nth number in the fibonacci sequence
#
# Returns an Integer
def fibonacci(n)
  ((PHI**n - I_PHI**n) / SQRT_5).round
end

fibonacci_spec.rb

require "rspec"
require "fibonacci"

# Bunch of tests to prove I'm right...
describe "#fibonacci" do

  it "returns 0 if x is 0" do
    expect(fibonacci(0)).to eql(0)
  end

  it "returns 1 if x is 1" do
    expect(fibonacci(1)).to eql(1)
  end

  it "returns 1 if x is 2" do
    expect(fibonacci(2)).to eql(1)
  end

  it "returns 2 if x is 3" do
    expect(fibonacci(3)).to eql(2)
  end

  it "returns 3 if x is 4" do
    expect(fibonacci(4)).to eql(3)
  end

  it "returns 5 if x is 5" do
    expect(fibonacci(4)).to eql(3)
  end
  
  it "returns 34 if x is 9" do
    expect(fibonacci(9)).to eql(34)
  end

end