Show that $F$ is a fibonacci number if and only if $5F^2 \pm 4$ is a perfect square.
i.e if one of $5F^2 + 4$ or $5F^2 - 4$ is a perfect square, then $F$ is fibonacci and vice-versa.
Show that $F$ is a fibonacci number if and only if $5F^2 \pm 4$ is a perfect square.
i.e if one of $5F^2 + 4$ or $5F^2 - 4$ is a perfect square, then $F$ is fibonacci and vice-versa.
