IF a Line be perpendicular to a Plane, all the Planes in which that Line shall be found, shall be perpendicular to the same Plane.
If the Line AB be perpendicular to the Plane ED; all the Plains in which it shall be found, shall be also perpen∣dicular to ED. Let AB be in the Plane AE, which hath for common section with the Plane ED, the Line BE, unto which let there be drawn the perpendicu∣lar FI.
Demonstration. The Angles ABI, BIF, are Right; thence the Lines AB, FI, shall be parallel; and (by the 8th.) FI shall be perpendicular to the Plane ED. So then the Plane AE shall be perpendicular to the Plane ED, (by the 5th. Def.)