立几课本中第33页11题: 经过一个角的顶点引这个角所在平面的斜线,如果斜线和这个角两边的夹角相等,那么斜线在平面上的射影是这个角的平分线所在直线. 立几课本中第122页第3题:AB和平面a所成角是θ1,AC在平面a内,AC和AB的射影AB’所成角θ2,设∠BAC=θ,求证:cosθ1·cosθ2=cosθ.(如图1) In a few textbooks, on page 33, 11 questions: After passing through the sloping line of the plane where the corner is located at the vertex of a corner, if the angle between the slash and the corner is equal, the projection of the slash on the plane is the bisector of the angle. In a straight line, a few textbooks on page 122. Problem 3: The angle formed by AB and plane a is θ1, AC is in plane a, and the angle θ2 formed by the projection AB′ of AC and AB is set by ∠BAC=θ. Cosθ1·cosθ2=cosθ. (Figure 1)