2. If two planes are perpendicular to each other, a straight line passing through a point in the first plane and perpendicular to the second plane is in the first plane.
3. If two intersecting planes are perpendicular to the third plane, then their intersection line is perpendicular to the third plane.
If two planes are perpendicular to each other, the perpendicular of one plane is parallel to the other.
Theorem for determining that a straight line is perpendicular to a plane (line-plane verticality theorem): If a straight line is perpendicular to two intersecting straight lines in a plane, it is perpendicular to the plane.
Inference 1: If one of two parallel lines is perpendicular to a plane, then the other line is also perpendicular to this plane.
Inference 2: If two straight lines are perpendicular to the same plane, they are parallel.
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