  We consider the scalar form factor in the weak current matrix element <P K|
j_\mu |0>, P =\pi,\eta,\eta'. It obtains the contributions from the scalar
meson resonance K^*_0(1430) and from the scalar projection of the vector meson
K^*(892) resonance. We analyze decay amplitudes of the Cabibbo suppressed
decays D->K P, P =\pi,\eta,\eta' using the factorization approach. The form
factors of the relevant matrix elements are described by assuming the dominance
of nearby resonances. The annihilation contribution in these decays arises from
the matrix element <P K| j_\mu |0>. All the required parameters are
experimentally known except the scalar meson K^*_0(1430) decay constant. We fit
the decay amplitudes and we find that final state interaction improves the
agreement with the experimental data. Then we extract bounds on scalar form
factor parameters and compare them with the experimental data obtained in the
analyses of K->\pi e\nu_e and K->\pi\mu\nu_\mu. The same scalar form factor is
present in the \tau-> K P \nu_\tau decay, with P =\pi,\eta,\eta'. Using the
obtained bounds we investigate the significance of the scalar meson form factor
in the \tau-> K P \nu_\tau, P = \pi, \eta, \eta' decay rates and spectra. We
find that the K^*_0(1430) scalar meson dominates in the \tau-> K \eta'\nu_\tau
decay spectrum.
