input: Write a function to convert a given temperature from Celsius to Fahrenheit. The function should take a temperature in Celsius as an argument and return the equivalent temperature in Fahrenheit. 
  
  assert celsius_to_fahrenheit(0) == 32
  assert celsius_to_fahrenheit(100) == 212
  assert celsius_to_fahrenheit(-40) == -40
  assert celsius_to_fahrenheit(20) == 68
  assert celsius_to_fahrenheit(-273.15) == -459.67
output: def celsius_to_fahrenheit(celsius):
      return celsius * 9/5 + 32