Hi Chao Peter Yang,
I guess the default plot range is not wide enough. Simply add the option PlotRange -> All like so:
Show[RevolutionPlot3D[{Sqrt[1 - (t - Sin[\[Pi]/5]/(2 Sin[(3 \[Pi])/10]))^2] - 1/2}, {t,
Sin[\[Pi]/5]/(2 Sin[(3 \[Pi])/10]), Sin[(72 \[Pi])/180]}, RevolutionAxis -> "X"],
RevolutionPlot3D[{Sqrt[1 - t^2]}, {t, Sin[(72 \[Pi])/180], 1}, RevolutionAxis -> "X"], PlotRange -> All]
Regards -- Henrik
