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三垂线定理及其逆定理的实质是斜线和它在平面上的射影同时垂直于平面内的某条直线,在学习这两个定理时应注意把握这一实质.
The essence of the triple vertical line theorem and its inverse theorem is that the slash and its projection in the plane are perpendicular to a certain line in the plane. We should pay attention to this essence when learning the two theorems.