In the quadrangular cone P-ABCD, PA⊥ plane ABCD, and then PAC⊥ plane ABCD.

If FG⊥AC passes through point F in △PAC, then FG⊥ surface ABCD.

FG/PA = CF/CP = CF/(CF+FP)= 1/8

FG= 1/8PA

And FG vertical plane ABCD,

Then FG is the height of the triangular pyramid F-BCD.