题目

写出从五个元素a、b、c、d、e中任取三个元素的所有组合，并求出其组合数. 答案：解析:考虑画出如下树形图,按给出字母从左到右的顺序来考虑.根据树形图，所有组合为abc,abd,abe,acd,ace,ade,bcd,bce,bde,cde.组合数为=10(个).小结:排列的树形图与组合的树形图是有区别的.排列的树形图中其元素不能重复出现但可任意排列，而组合的树形图中其元素也不能重复出现，但元素出现的次序必须按照There are good Santas and there are bad Santas. The good Santas all realize one thing: In the mind of a child, Santa Claus is perfect. That’s difficult. And so they come from around the world to attend the Charles W. Howard School in Midland, Mich. The school offers a three-day, 40-hour course for Santas hoping to be excellent. There’s a real range to what Santas earn, from big bucks for large events to no pay for charities①. One thing for them all is a pure love for children. A former Macy’s Santa named Charles Howard founded the Michigan school in 1937. “Many people will think if you have a suit and you’re a Santa,” said Tom Valent, the school’s current dean. “It’s more than that.” So with a pet reindeer②, his properly named wife Holly and a workshop full of wonder. The school teaches everything from breathing techniques for carol singing to the beard maintenance. There are lessons in child psychology, sign language and even media training for talk-show appearances and call-in shows. Santas also need to make sure they know all the names of the reindeer. What’s harder to deal with, however, is children who ask for Christmas gifts like bringing daddy home or making mommy happy again. After spending time in the classroom, Weir went to the mall to test his training. However, at one point, his hat fell off, and he was unsure how to react. “You’re the real Santa,” Valent said. “Pick that hat up, slip it back on, keep smiling and do your job. You’ve got to stay on top of your game. Even the last one at night, when you’re tired, you’ve got to remember: This one’s forever, so do a good job.” Notes: charity  n. 慈善，慈善团体 reindeer  n. 驯鹿 According to the passage, good Santas should be the ones that ________.         A. can offer the children some gifts        B. help adults look after their children         C. have ability to make children happy     D. should be perfect in the mind of children The underlined words “big bucks” should refer to ________.         A. lots of money   B. many gifts    C. loves for children    D. gifts from children The Michigan school was founded by Charles Howard with the purpose to ________.         A. train the pet reindeer for the Santas       B. earned money from its students         C. train perfect Santas for the world         D. show the world’s concern to the poor According to Weir’s performance, we can conclude that ________.         A. Weir was short of experience            B. Weir was not fit for the job as a Santa         C. Weir did very badly in the game         D. Weir was not a good student in school