题目

某班名学生某次数学考试成绩（单位：分）的频率分布直方图如图所示．（学生成绩都在之间） （1）求频率分布直方图中的值； （2）估算该班级的平均分； （3）若规定成绩达到80分及以上为优秀等级，从该班级40名学生中任选一人，求此人成绩为优秀等级的概率． 答案：解:(1)由题,,          --------2分 ∴,                                                -------- 4分 （2）该班级的平均分为76.5----9分 （3）此人成绩为优秀等级的概率为0.4                          …… 14分29、The word “day” has two meanings. When we talk about the number of days in a year, we are using “day” to mean 24 hours. But when we talk about day and night, we are using “day” to mean the time between sunrise and sunset. Since the earth looks like a ball, the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from night into day over and over by the spinning（旋转） of the earth. At the equator（赤道） day and night are sometimes the same length. They are each twelve hours long. The sun rises at 6 in the morning and sets at 6 in the evening. For six months the North Pole is tilted（倾斜） toward the sun. In those months the Northern Hemisphere（半球） gets more hours of sunlight than the Southern Hemisphere. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than night. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days. 1. When the Western Hemisphere is having day, the Eastern Hemisphere is having ______. A. both day and night      B. day             C. neither day nor night       D. night 2. A place is moved from day into night and from night into day over and over by ______ of the earth. A. the pushing           B. the pulling        C. the spinning        D. the passing 3. At the equator day is as long as night ______. A. sometimes            B. never            C. usually             D. always 4. When the North Pole is tilted toward the sun, the Northern Hemisphere gets _____ sunlight. A. less                  B. more             C. all                 D. no 5. When it is winter in China, ______. A. the USA is tilted toward the sun           B. the South Pole is tilted away from the sun C. the North Pole is tilted toward the sun      D. the North Pole is tilted away from the sun