           
    1 1 5 9 13 17          2 0.625 1.25 2.5 5    
                                      
   1     4         21   17+4  
    2          2       10   5 2  
                             
     10                 10          
              n   n-1 n-2       
  n   an      n            n-1        an-1       n-2       n    an-2          an= an-1+an-2 
   a1=1 a2=2            a1=1 a2=2 a3=a1+a2=3 a4=a2+a3=5       8 13 21 34 55 89   a10=89  10    89         an-1 an-2an       