2015年2月,安徽省淮北市和亳州市两市联合举行了高三第一学期期末联考,试卷中有一道填空题,看似平淡,实则构思奇巧,入口较宽,很有厚重感,值得细细品味.题目如图1,已知正方体ABCDA1B1C1D1的棱长为2,M、N分别为AD、CC1的中点,O为上底面A1B1C1D1的中心,则三棱锥O-MNB的 February 2015, Huaibei City, Anhui Province and Bozhou City jointly held the first semester of the third year of high school entrance exam, there is a fill in the papers questions, seemingly plain, but ingenious, wide entrance, a sense of weight, it is worth Savor taste.The subject is shown in Figure 1, the known cube ABCDA1B1C1D1 the edge length of 2, M, N respectively AD, the midpoint of CC1, O is the center of the upper surface A1B1C1D1, then the triangular pyramid O-MNB