  There is an equation relating numbers of curves on F_0 satisfying incidence
conditions and numbers of curves on F_2 satisfying incidence conditions. The
purpose of this paper is to give a tropical proof of this equation in the case
of rational curves. We use induction on the degree and two Kontsevich-type
formulas for curves on F_0 and on F_2. The formula for F_2 was not known before
and is proved using tropical geometry.
