2024 AMC 10B Problem 23

Below is the professionally curated solution for Problem 23 of the 2024 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2024 AMC 10B solutions, or check the answer key.

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Concepts:Fibonaccisummation

Difficulty rating: 2270

23.

The Fibonacci numbers are defined by F1=1,F_1 = 1, F2=1,F_2 = 1, and Fn=Fn1+Fn2F_n = F_{n-1} + F_{n-2} for n3.n \ge 3. What is

F2F1+F4F2+F6F3++F20F10?\frac{F_2}{F_1} + \frac{F_4}{F_2} + \frac{F_6}{F_3} + \cdots + \frac{F_{20}}{F_{10}}?

318318

319319

320320

321321

322322

Solution:

Use F2k=FkLk,F_{2k} = F_k L_k, so each term F2kFk=Lk,\dfrac{F_{2k}}{F_k} = L_k, the kkth Lucas number. That collapses the sum to k=110Lk.\sum_{k=1}^{10} L_k. With L1=1,L2=3,L3=4,,L10=123,L_1 = 1, L_2 = 3, L_3 = 4, \ldots, L_{10} = 123, the identity k=1nLk=Ln+23\sum_{k=1}^{n} L_k = L_{n+2} - 3 gives L123=3223=319.L_{12} - 3 = 322 - 3 = 319. Thus, B is the correct answer.

Problem 23 in Other Years