Exercise 1.8: Newton’s method for cube roots is
based on the fact that if 
  y
 is an approximation to the cube root of 
  x
,
then a better approximation is given by the value


  
    
      
        
          x
          
            /
          
          
            y
            2
          
        
        +
        2
        y
      
      3
    
    .
  


Use this formula to implement a cube-root procedure analogous to the
square-root procedure.  (In 1.3.4 we will see how to implement
Newton’s method in general as an abstraction of these square-root and cube-root
procedures.)