  The frequency ratios and of HD 49933 exhibit an increase at high frequencies.
This behavior also exists in the ratios of other stars, which is considered to
result from the low signal-to-noise ratio and the larger line width at the
high-frequency end and could not be predicted by stellar models in previous
work. Our calculations show that the behavior not only can be reproduced by
stellar models, but can be predicted by asymptotic formulas of the ratios. The
frequency ratios of the Sun, too, can be reproduced well by the asymptotic
formulas. The increased behavior derives from the fact that the gradient of
mean molecular weight at the bottom of the radiative region hinders the
propagation of p-modes, while the hindrance does not exist in the convective
core. This behavior should exist in the ratios of stars with a large convective
core. The characteristic of the ratios at high frequencies provides a strict
constraint on stellar models and aids in determining the size of the convective
core and the extent of overshooting. Observational constraints point to a star
with $M=1.28\pm0.01 M_{\odot}$, $R=1.458\pm0.005 R_{\odot}$, $t=1.83\pm0.1$
Gyr, $r_{cc}=0.16\pm0.02 R_{\odot}$, $\alpha=1.85\pm0.05$, and
$\delta_{ov}=0.6\pm0.2$ for HD 49933.
