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1.课本第27页25节定理:在平面内的一条直线,如果过这平面的一条斜线的斜线足,并且与这斜线的射影垂直,那末它也与这斜线垂直(即三垂线定理).其逆定理:在平面内的一条直线,如果过这平面的一条斜线的斜线足,并且与这斜线垂直,那末它也与这斜线的射影垂直.其中“过这平面的一条斜线的斜线足”一语,假如删去也可用同样的方法来证明这修改了的定理.证明的过程也不感到困难或复杂,并且在记忆和应用时,反而感到方便,使用范围也较广.譬如习题三
1. Textbook Page 27 Section 25 Theorem: A straight line in the plane, if the diagonal line of a slash across this plane is sufficient and perpendicular to the projective of this slash, then it is also perpendicular to this slash (ie, three (The vertical line theorem.) The inverse theorem: A straight line in a plane, if the slash of a slash across this plane is sufficient and perpendicular to this slash, then it is also perpendicular to the projective of this slash. This slashed diagonal line of the plane is the same. If the deletion is used, the same theorem can be used to prove the modified theorem. The process of proof is not difficult or complex, and it is convenient to memorize and apply it. The use of a wide range. For example, exercise three