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# Fibonacci sequence the Golden Ratio

The Fibonacci sequence is the sequence defined by:

First 15 sequence values:

n01234567891011121314
Fn01123581321345589144233377

The number

$\phi =\frac{1+\sqrt{5}}{2}=1.61803...$

is called the Golden Ratio. It satisfies the property

$\phi =1+\frac{1}{\phi }$

Can you show that the sequence of ratio of successive Fibonacci numbers

$\frac{{F}_{n+1}}{{F}_{n}}$

converges to the Golden Ratio?

$\underset{n\to \infty }{\mathrm{lim}}\frac{{F}_{n+1}}{{F}_{n}}=\phi$

Observe that

${F}_{14}/{F}_{13}=377/233\approx 1.618025$

is already pretty close to φ.

What conditions must be met for this ratio to converge?

Extension: Missing Squares

An elegant variation of this idea makes use of two rectangles of such proportion that they fit side by side to make a perfect 8 by 8 checkerboard. When the pieces are rearranged to make the larger rectangle, there is an apparent gain in area of one square unit.  Where did the extra square come from, and what does this have to do with Fibonacci?

# Pythagorean Triples

Pythagorean triples are sets of three integers that are solutions to the associated identity for right triangles.

The question is this:

Can you find a formula or algorithm for generating Pythagorean Triples?

There may be more than one formula, and each might not be exhaustive. All we are after is a set of rules such that, given one number, can you find the other two that belong to the triplet?

# Infinitude of Primes

Euclid’s second theorem is simply stated:

The number of primes is infinite.

Can you show this to be true?

Bonus:  Can you prove it without using a contradiction?

# Probability of Coprimality

This one is rather easy to pose, and can be difficult to solve, even with a proof sitting right in front of you. The result is a bit surprising as well, and gives some insight into the structure of the integers and prime numbers.

Given two random integers, what is the probability that they have no common factors besides 1?

or, how it is usually posed:

What is the probability of two random integers being coprime?

Where “coprime” means “the greatest common divisor is one”.

# Ten Digit Number

Is there a 10-digit number where the first digit is equal to how many 0’s are in the number, the second digit is equal to how many 1’s are in the number, the third digit is equal to how many 2’s are in the number, all the way up to the last digit, which is equal to how many 9’s are in the number?

# Common Fractions & Decimal Equivalents

Step 1:  Use long division (no calculators please!) to convert the fractions (in this case, specifically reciprocal integers) to decimal representations.

Step 2:  Look at the results.  What do you notice?  Write down all the questions that come up.

Step 3:  As a group, we will make a list of all the questions.

Step 5:  Discuss!

Note:  Work with a partner to divide up the work in Stages 1, 2, and 4.

1/2 = __________                         1/11 = __________

1/3 = __________                         1/12 = __________

1/4 = __________                         1/13 = __________

1/5 = __________                         1/14 = __________

1/6 = __________                         1/15 = __________

1/7 = __________                         1/16 = __________

1/8 = __________                         1/17 = __________

1/9 = __________                         1/18 = __________

1/10 = __________                        1/19 = __________

1/20 = __________