解题方法
1 . 甲、乙、丙三人以正四棱锥和正三棱柱为研究对象,设棱长为
,若甲从其中一个底面边长和高都为2的正四棱锥的5个顶点中随机选取3个点构成三角形,定义随机变量
的值为其三角形的面积;若乙从正四棱锥(和甲研究的四棱锥一样)的8条棱中任取2条,定义随机变量
的值为这两条棱的夹角大小(弧度制);若丙从正三棱柱的9条棱中任取2条,定义随机变量
的值为这两条棱的夹角大小(弧度制).
(1)比较三种随机变量的数学期望大小;(参考数据
)
(2)现单独研究棱长
,记
(
且
),其展开式中含
项的系数为
,含
项的系数为
.
①若
,对
成立,求实数
,
,
的值;
②对①中的实数
,
,
用数字归纳法证明:对任意
且
,
都成立.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b6a24198bd04c29321ae5dc5a28fe421.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f022950e0faa45b617d497b01b5292b9.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5b734e8f1546481e3eb4976008a045de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/47f5817ab7b88e9d8a83dd086ffdb3c2.png)
(1)比较三种随机变量的数学期望大小;(参考数据
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4d854a36c484c5e5400b11384dd120ea.png)
(2)现单独研究棱长
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b6a24198bd04c29321ae5dc5a28fe421.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/40c2ea763797a470a849899851c01d12.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0704f453b2de48d36911f7db496bbf82.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/930bc56406e69b785b37a83d48e36724.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/81dea63b8ce3e51adf66cf7b9982a248.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/08eb71ecf8d733b6932f4680874dbbf3.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/9b0a89e3c30f6e4d4c5db4378b05d987.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f1ae9a3b0b7aeb1545b65d91aa371b3c.png)
①若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/048651e049071a622651832e6446a75e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2d8978869e64ccf247c75fc6a3c71981.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0a6936d370d6a238a608ca56f87198de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2c94bb12cee76221e13f9ef955b0aab1.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/071a7e733d466949ac935b4b8ee8d183.png)
②对①中的实数
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0a6936d370d6a238a608ca56f87198de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2c94bb12cee76221e13f9ef955b0aab1.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/071a7e733d466949ac935b4b8ee8d183.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0704f453b2de48d36911f7db496bbf82.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/930bc56406e69b785b37a83d48e36724.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/048651e049071a622651832e6446a75e.png)
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