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Courseware of "The Three Perpendicular Theorem" (Higher Grade 3 of Shanghai Education Edition) 1. The theorem for determining the perpendicularity of a straight line and a plane is 2. â‘ A vertical line is drawn to this plane through a point outside the plane. The vertical foot is called this point in this plane. â‘¡ A straight line intersects a plane, but is not perpendicular to this plane, then this straight line is called this plane. â‘¢Draw a vertical line from a point that is oblique enough to the outer side to the plane, and the line passing through the vertical foot and diagonal foot is called