由题意知：∠NPB=∠NMC=α．
Rt△MNC中,MC=n,∠NMC=α,
∴NC=MC•tanα=n•tanα,
∴BN=BC-NC=m-n•tanα．
Rt△BPN中,∠BPN=α,
∴BP=BN÷tanα=(m-n•tanα)/ tanα．