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新课标苏教选修2—1教材第98页习题第9题要我们求证三余弦定理:已知平面的一条斜线和它在平面内的射影的夹角为θ_1,平面内的一条直线和这条斜线在平面内的射影的夹角为θ_2.设斜线和平面内这条直线的夹角为θ,则cosθ=cosθ_1·cosθ_2.
The new curriculum standard Elective Course for Jiangsu Electives 2-1 Teaching material page 98 Exercise 9 Question We need to verify the triple cosine theorem: The angle between a slash of the known plane and its projection in the plane is θ_1, a straight line in the plane The angle between the projective line and this oblique line in the plane is θ_2. Let the angle between the oblique line and the line in the plane be θ, then cosθ = cosθ_1 · cosθ_2.