In our notation we have that $F_{n-1}$ is the $n$th Fibonacci number since we start with $F_{0}=1$.
We want to prove that every third Fibonacci number is divisible by $F_{2}=2$.
The proof is the following using induction:
$F_{3n+2}=F_{3n+1}+F_{3n}$
$F_{3n+2}=F_{3n}+F_{3n-1}+F_{3n-1}+F_{3n-2}$ (A)
$F_{3n+2}=F_{3n-1}+F_{3n-2}+F_{3n-1}+F_{3n-2}$ (B)
$F_{3n+2}=2(F_{3n-1}+F_{3n-2})$
I don't understand how you go from step (A) to step (B) can anyone explain this to me?
Best Answer
There's a missing term in (B) as you noticed. A correct proof is by induction. Show that $F_2$ is even (immediate). Then write
$$ \begin{align} F_{3n+2} &= F_{3n+1} + F_{3n} \\ &= 2F_{3n} + F_{3n-1} \\ &= 2F_{3n} + F_{3(n-1)+2} \enspace, \end{align} $$
which is even because it's the sum of two even numbers. ($F_{3(n-1)+2}$ is even by the induction hypothesis.)