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《立体几何》第一章第27页有这样一个结论：斜线和平面所成的角，是这条斜线和平面内经过斜足的直线所成的一切角中最小的角。
There is a conclusion in the first chapter of the “Three-Dimensional Geometry” on page 27 that the angle between the slash and the plane is the smallest of all the angles between the slash and the diagonal line in the plane.