# Fibonacci Numbers: Recreational math

Reference: ICEEM Year 8 Math text book, Australian Mathematical Society.

A Fibonacci sequence is a sequence $F_{1}, F_{2}, F_{3}, \ldots$ of numbers in which each term from the third one onwards is the sum of the two terms that immediately precedes it. You have to have two numbers to start with, $F_{1}$ and $F_{2}$. These are called seeds. Then,

$F_{3}=F_{1}+F_{2}$

$F_{4}=F_{2}+F_{3}$

and so on. The classic Fibonacci sequence has 1 and 1 as its seeds. It first ten terms are:

1,1,2,3,5,8, 13, 21, 34 and 55.

Use a calculator when appropriate in the following:

Activity 1

Write out the classic Fibonacci sequence as far as its 25th term, $F_{25}$. Before you calculate $F_{11}$, make a rough guess of what $F_{25}$ will be. See how good your guess turns out to be.

Activity 2

Pick any two numbers as seeds and work out the first 20 terms for that Fibonacci sequence. Pick entirely different seed numbers from the person besides you (example, your friend, or teacher, or any one else imaginary :-)), and keep your list reasonably neat, as we will be coming back to it in a little while.

Activity 3

Swap your two seed numbers from Activity 2 around and figure out the first twenty terms in the new Fibonacci sequence. (If, your sequence in Activity 2 started 6, 11, 17, 28, 45, …) your new sequence will start 11, 6, 17, 23, 40, ….). Yes, you do get quite different numbers from Activity 2.

Activity 4:

It is now time to make a few observations about your Fibonacci sequences.

• The classic sequence in Activity 1 has two odd seeds. This gives a certain pattern of odd and even terms through out the sequence. What happens if you start with two even seeds, or an odd and even seed? Explain.
• Compare the 10th terms you generated in each of the sequences in Activity 2 and Activity 3. Which one is larger? Compare the 20th term as well. Can you explain what is happening?
• Use the calculator to divide the term $F_{10}$ by the term $F_{9}$ immediately before it. Write your answer down. Then, do the same with the second sequence. Now repeat the calculation with terms $F_{20}$ and $F_{19}$. Do you notice anything interesting? Did any of the “other people” who are doing this activity get the same ratio? They should all have got the same ratio. They should all get the same number, even though there may be v small diferences in the sixth decimal places.
• For the classic Fibonacci sequence, the first two terms larger than 1000000 are $F_{32}=1346269$ and $F_{33}=2178309$. Use these two values to see if what you noticed in the previous ratio calculations still holds for higher order terms in the classic Fibonacci sequence.

The number you obtained (to a good approximation) in the ratio calculations is famous and interesting enough to deserve its own Greek letter. It is called $\phi$ (pronounced to rhyme with spy). It is called the golden ratio or golden mean. It is a very interesting number with long history.

Search Google and you will discover some amazing facts about $\phi$. It appears in many different ways in geometry and architecture.

Now try calculating these values and see what you notice about them:

a. $\phi^{2}$

b. $\frac{1}{\phi}$

c. $(2\phi -1)^{2}$

Regards,

Nalin Pithwa.

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