Exercise 1.39: A continued fraction
representation of the tangent function was published in 1770 by the German
mathematician J.H. Lambert:


  
    tan
    ⁡
    x
  
  
  =
  
  
    
      x
      
        1
        −
        
          
            x
            2
          
          
            3
            −
            
              
                x
                2
              
              
                5
                −
                …
              
            
          
        
      
    
    
    ,
  


where 
  x
 is in radians.  Define a procedure (tan-cf x k) that
computes an approximation to the tangent function based on Lambert’s formula.
k specifies the number of terms to compute, as in Exercise 1.37.