Plane BDD 1 vertical packaging

PB 1 is in the plane BDD 1.

The recording plane BDD 1 and AC intersect at point O.

Just prove that B 1P is perpendicular to PO.

That is, angle b 1po = 90.

In other words, triangle B 1D 1P is similar to triangle PDO.

b 1d 1/PD = d 1P/DO

Obviously b1d1/PD = d1p/do.