新教材第二册(下B)P43,“平面的斜线和平面所成的角”一节,为引入最小角定理,在图1中证明了公式:cosθ=cos1θcos2θ.图1图2这里1θ是平面α的斜线OA和它在平面内射影AB所成的角,AC是α内任一直线,AB和AC所成的角为θ2,OA和AC所成角为θ,图中BC⊥AC.本节的练习题2是:已知平面的 The second volume of the new textbook (bottom B) P43, “The angles formed by the slashes and the planes of the plane”, is the introduction of the minimum angle theorem. The formula is proved in Figure 1: cosθ = cos1θcos2θ. Figure 1 Figure 2 Here 1θ Is the angle formed by the diagonal OA of the plane α and its projection AB in the plane, AC is any straight line within α, the angle formed by AB and AC is θ2, the angle formed by OA and AC is θ, and BC is shown in the figure. AC. Exercise 2 in this section is: Known planar