题目

【2018江苏南京师大附中、天一、海门、淮阴四校高三联考】如图，一只蚂蚁从单位正方体的顶点出发，每一步（均为等可能性的）经过一条边到达另一顶点，设该蚂蚁经过步回到点的概率．（I）分别写出的值；（II）设顶点出发经过步到达点的概率为，求的值；（III）求． 答案：【答案】（I）；（II）；（III）．【解析】试题分析：（1）由题意得经过1步不可能从点A回到点A，故；经过2步从点A回到点A的方法有3种，即A-B-A；A-D-A；，且选择每一种走法的概率都是，由此可得所求概率．（2）分为奇数和偶数两种情况讨论可得结论．（3）结合（2）中的结论，分四种情况可得，又，二、句型转换1.Mr. Wang is going to take his son to school.(改为否定句)Mr. Wang ______ ______ ______ take his son to school.2.He will bring you some storybooks.(改为一般疑问句)______ ______ bring you ______ storybooks?3.The students are swimming in the river now. (用tomorrow改写句子)The students ______ ______ in the river tomorrow.4.Where are you going this Saturday?(改为同义句)Where ______ you ______ this Saturday?5.They are going to have a sports meeting next week.(对画线部分提问)______ ______ they going to have a sports meeting?