Key words in question (Fibonacci series, infinite series, in the middle of the question one number series is there. I got the series 3 12 7 26 15 b?

• ans 54
  3,12,7,26,15
  3*2=6+1;
  12*2=24+2;
  7*2=14+1;
  26*2=52+2=54.
  
• 10 years agoHelpfull: Yes(20) No(5)
• as it is 3 12 7 26 15 b
  here the differnce between 3&7 is 4 and The difference between 7&15 is 8 i.e. the differnce between the no. is doubled.so similarly the differnce between 12 &26 is 14 so the nxt difference should be 28 . The answer is 54
• 10 years agoHelpfull: Yes(5) No(1)
• 3- 12 - 7- 26 - 15
  / / / /
  9 -5 19 -11
  / / /
  -14 14 -30
  
  See the third level of difference its alternating positive and negative number so the next in the list would be 30 for the third level and 19 for the second level and 36 for the first level
  i.e b=36
• 10 years agoHelpfull: Yes(0) No(5)
• it will go 3 12 15 ...
  12 26 ....
  so,
  3
  3+3+1=7
  7+7+1=15
  12
  12+12+2=26
  26+26+4=54
• 10 years agoHelpfull: Yes(0) No(0)
• IF the series is 3, 12, 7, 26, 15, b
  
  We get two different series: 3, 7, 15 and 12, 26, b.
  
  In first series, 3x2 + 1 = 7. 7x2+1= 15 and so on.
  
  In second series, 12x2 + 2 =26. So, b is 26x2 +2 =54.
• 6 years agoHelpfull: Yes(0) No(0)