input: Write a python function to calculate the roots of a quadratic equation.
  assert quadratic_roots(1, -3, 2) == (2.0, 1.0)
  assert quadratic_roots(1, 4, 4) == (-2.0, -2.0)
  assert quadratic_roots(1, -6, 9) == (3.0, 3.0)
output: import math
  
  def quadratic_roots(a, b, c):
      discriminant = b**2 - 4*a*c
      if discriminant >= 0:
          root1 = (-b + math.sqrt(discriminant)) / (a * 2)
          root2 = (-b - math.sqrt(discriminant)) / (a * 2)
          return (root1, root2)
      else:
          return None