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例1(2008年全国卷Ⅰ理11题)已知三棱柱ABC-A_1B_1C_1的侧棱与底面边长都相等,A_1在底面ABC内的射影为△ABC的中心,则AB_1与底面ABC所成角的正弦值等于
Example 1 (2008 national volume I theory 11) It is known that the side and bottom side lengths of the triangular prism ABC-A_1B_1C_1 are all equal. The projection of A_1 on the bottom surface ABC is the center of △ABC, and the angle between AB_1 and the bottom surface ABC. The sine value is equal to