## Invitation to Computer Science 8th Edition

Counting "by hand" leads us to $F(20)=6567$ --- Computing the value of $F(20)$ using just the definition of the Fibonacci the sequence would mean that we just have to write down the sequence up to the $20^{t h}$ element. So using the knowledge: $F(1)=1$ $F(2)=1$ $F(n)=F(n-1)+F(n-2)$ for $n>2$ we get: $1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181,6765$ resulting $F(20)=6765$