Three perpendicular inverse theorem

Theorem .

If the straight line on the plane is perpendicular to the oblique, then it is also perpendicular to the projection of the oblique.



Proof .

Similar to the three perpendiculars theorem, if a straight line with perpendicular to inclined CA, then, being perpendicular to and straight line CA`, is perpendicular to the plane β, and hence to the projection of inclined BC. The theorem is proved.