input: Write a function to convert a given temperature value from Fahrenheit to Celsius and round the result to two decimal places. If the input temperature is None, return None.
  assert fahrenheit_to_celsius(32) == 0.0
  assert fahrenheit_to_celsius(212) == 100.0
  assert fahrenheit_to_celsius(98.6) == 37.0
  assert fahrenheit_to_celsius(-40) == -40.0
  assert fahrenheit_to_celsius(None) == None
output: def fahrenheit_to_celsius(temp):
      if temp is None:
          return None
      return round((temp - 32) * 5 / 9, 2)