It is the idea that the two planes are at right angles.

FIRST for a

THEN if another plane contains that line then the two planes are perpendicular.

FIRST for a

**line**to be perpendicular to a plane it must be at right angles to all lines on the plane that intersect it.THEN if another plane contains that line then the two planes are perpendicular.

Copyright © 2018 MathsIsFun.com