应用面积射影公式求二面角的大小 ,对于 (一 )平面角虽可作出 ,但比较困难 ,计算繁琐 ;(二 )平面角无法作出 ,或很难下手 .如 :1.直三棱柱ABC-A1 B1 C1 中 ,∠BAC=90° ,AB =BB1 =1,直线B1 C与平面ABC成30°角 ,求二面角B -B1 C -A的余弦值 .解 :易知∠BCB1 =30° ,作AD⊥BC于D The area projective formula is used to find the size of the dihedral angle. Although (a) the plane angle can be made, it is difficult and the calculation is cumbersome; (b) The plane angle cannot be made, or it is difficult to start. For example: 1. Straight triangular prism ABC- In A1 B1 C1, ∠BAC=90° and AB=BB1 =1, the straight line B1 C and the plane ABC are at an angle of 30°. Find the cosine of the dihedral angle B - B1 C -A. Solution: 易 BCB1 = 30 ° for AD BC in D