**2.3.2 Kaon and pion decay constants**

The kaon and pion decay constants can also be used to determine Vus. Since the ratio fK/fπ is related to Vus/Vub, we can obtain Vus if Vub is precisely known. From these quantities, we calculate the two point function of axial vector current and pseudo-scalar operator in the same way as the form factor.
