Senior two mathematics textbook

And ∠ABC, there is a small P outside ABC.

The intersection point P is defined as PO⊥ surface ABC, OE⊥AB, OF⊥BC, and the vertical legs are E and F respectively, connecting PE and PF. According to the theorem of three perpendicular lines, PE⊥AB,PF⊥BC, in △PBE and △PBF, ∠ PEB =

That is, the projective BO of PB on the plane ABC is the bisector of ∠ABC.

Sorry, the picture will not be uploaded, I hope you can understand.