Giải thích các bước giải:
Ta có $AM//BN$
$\to \dfrac{CM}{CB}=\dfrac{AM}{BN}$
Lại có $AM//CP$
$\to \dfrac{BM}{BC}=\dfrac{AM}{CP}$
$\to \dfrac{AM}{BN}+\dfrac{AM}{CP}=\dfrac{CM}{CB}+\dfrac{BM}{BC}=\dfrac{CM+BM}{BC}=\dfrac{BC}{BC}=1$
$\to \dfrac1{BN}+\dfrac1{CP}=\dfrac1{AM}$