Exercise 4 Page 313

The Italian mathematician Leonardo Fibonacci, known also as Leonardo Pisano and Leonardo of Pisa, is best known for the recursive sequence shown in Example $2,$ defined by the recursive rule $a_n=a_{n-1}+a_{n-2}.$ Its first terms are $a_1=1$ and $a_2=1.$

This is why this sequence is also known as The Fibonacci Sequence. He used it to describe how pairs of rabbits increase as they multiply under the conditions described in Example $1.$ It was later discovered that this sequence is related to the golden ratio. It appears in nature, for example, in the spirals of sunflower heads.