The sum of first 40 terms 1,6,7,13,20 .... ?

• There is actually 2 series. 1. (1,7,13,...upto 20 terms) and 2. (6,13,20,..., upto 20 terms)
  1st series has a=1,n=20,d=6 and 2nd series has a=6,n=20,d=7.
  Sum of 1st serires=1160 and sum of 2nd series=1450.
  
  Hence the required answer=1160+1450=2610
• 9 years agoHelpfull: Yes(7) No(13)
• it is a fibonacci series.do anyone know the formula of sum of fibonacci series????
• 9 years agoHelpfull: Yes(5) No(1)
• i think it is not a valid series it should be 1,7,13,20,33,53
• 9 years agoHelpfull: Yes(2) No(5)
• 40th term can be found from =(1.61803)^40-(-1.61803)^40/root5
  now we can use the formula=n/2(a+l)
• 9 years agoHelpfull: Yes(1) No(0)
• Phi ^40/sqrt(5) = 102334154.99999999
  -(-phi)^40/sqrt(5) =-1.9543817017886194e-9
  Fib(40) = 102334154.99999999
• 9 years agoHelpfull: Yes(1) No(0)