The form factors in the semileptonic decays of heavy $B/D$ meson decays can be represented by dispersion relations, by which the kinematics in the decay region are related to the ones in the scattering region. By fitting to the lattice QCD data on the form factors, the subtraction constants in their dispersive representations can be determined such that the form factors in the whole kinematical region are obtained. Cabibbo–Kobayashi–Maskawa elements can be extracted with form-factor values at $q^2=0$.