Goi $I=BD\cap RQ$trong mp $(ABD)$$ :S=AD\cap PI\Rightarrow S=AD\cap (PRQ)$
ap dung dinh li menelauyt vao $\triangle CBD (I,Q,R$ thang hang$)$:
$\frac{ID}{IB}.\frac{RB}{RC}.\frac{QC}{QD}=1\Rightarrow \frac{ID}{IB}.2.1=1\Rightarrow \frac{ID}{IB}=\frac{1}{2} $
$\triangle ABD(P,S,I$ thang hang$):$
$\frac{ID}{IB}.\frac{PB}{PA}.\frac{SA}{SD}=1\Rightarrow \frac{1}{2}.1.\frac{SA}{SD}=1\Rightarrow SA=2SD$(dpcm)