  A phenomenological analysis of $D\to K\pi$ and $D_s^+\to K K $decays
including both Cabibbo-favored and doubly-Cabibbo-suppressed modes have been
presented by employing the present experimental data. SU(3) symmetry breaking
effects from the decay constants and form factors have been taken into account
in the analysis. Three asymmetries, $R(D^0)$, $R(D^+)$, and $R(D_s^+)$, which
are generated through interference between Cabbibo-favored and doubly-
Cabibbo-suppressed decays, are estimated. Theoretical results agree well with
the current measurements.
