Compute 2 1 + 4 1 + 8 2 + 1 6 3 + 3 2 5 + ⋯ + 2 k F k + … where F k represents the the k th term of the Fibonacci sequence: 1 , 1 , 2 , 3 , 5 , 8 , 1 3 , . . .
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Very nice! But you need to prove that S is finite else you would be performing arithmetic on infinities.
I solved the problem this way. By the way, how does this problem have any calculus at all?
Series is a calculus topic but you don't have to use calculus to solve it. Also, I saw this exact same problem posted by someone else, and that problem is Level 2, but this is Level 4. There's some discrepancy here.
Yes indeed. Very famous problem from the Inequalities Chapter of Problem Solving Strategies.
We can also use generating functions for this problem.
Letting x = 2 1
S = x + x 2 + 2 x 3 + 3 x 4 + 5 x 5 + 8 x 6 + 1 3 x 7 + 2 1 x 8 + 3 4 x 9 + . . .
x ∗ S = 0 x + 1 x 2 + 1 x 3 + 2 x 4 + 3 x 5 + 5 x 6 + 8 x 7 + 1 3 x 8 + 2 1 x 9 + . . .
x 2 ∗ S = 0 x + 0 x 2 + 1 x 3 + 1 x 4 + 2 x 5 + 3 x 6 + 5 x 7 + 8 x 8 + 1 3 x 9 + . . .
Now: ( 1 − x − x 2 ) S = x
S = 1 − x − x 2 x
Substituting x = 2 1 , we get S = 2
S = n = 1 ∑ ∞ ( 5 1 ⋅ ( 4 1 + 5 ) n − 5 1 ⋅ ( 4 1 − 5 ) n )
Let φ 1 = 4 1 + 5 and φ 2 = 4 1 − 5
Then S = 5 1 ⋅ ( ( 1 − φ 1 ) φ 1 − ( 1 − φ 2 ) φ 2 ) = 5 1 ⋅ ( 3 − 5 1 + 5 − 3 + 5 1 − 5 )
Therefore S = 5 1 ⋅ ( 2 + 5 − ( 2 − 5 ) ) = 2
great solution @AnatoliyRazin .... but how did that complicated factorization came into your mind
It's called Binet's formula for the Fibonacci sequence
I did it this way. It isn't complicated. Anybody who has studied algebra in depth will come across the closed form for the fibonacci sequence and try to prove it. It was a result that stuck and reused. However I like Dhruva's solution more because surds introduce mistakes very easily.
Problem Loading...
Note Loading...
Set Loading...
S = 2 1 + 4 1 + 8 2 + 1 6 3 + 3 2 5 + 6 4 8 . . . S = 2 1 ( 1 + 2 1 + 4 2 + 8 3 + 1 6 5 + 3 2 8 . . . ) 2 S = 1 + 2 1 + 4 2 + 8 3 + 1 6 5 + 3 2 8 . . . 2 S − S = 1 + 4 1 + 8 1 + 1 6 2 + 3 2 3 . . . S = 1 + 2 1 ( 2 1 + 4 1 + 8 2 + 1 6 3 + 3 2 5 + 6 4 8 . . ) S = 1 + 2 S 2 S = 1 S = 2