a) Ta có: $MB=AB+MA=3+1=4\mathit{(cm)}$
Xét $\Delta MBP$ có: \(AD//BC\) (gt)
\(\Rightarrow \frac{AD}{BP}=\frac{MA}{MB}\) (đli Talet đảo)
\(\Rightarrow BP=\frac{AD.MB}{MA}=\frac{2.4}{1}=8\mathit{(cm)}\)
b) Xét $\Delta NPC$ có: \(AD//PC\) (vì \(AD//BC;P\in BC\))
\(\Rightarrow \frac{NC}{NA}=\frac{PC}{AD}\) (đli Talet đảo)
Khi đó: \(\frac{MA}{MB}.\frac{PB}{PC}.\frac{NC}{NA}=\frac{AD}{BP}.\frac{PB}{PC}.\frac{PC}{AD}=1\) (đpcm)
$\text{#}Toru$