题目

在体对角线长为l的长方体中,体对角线在相邻三个面上的射影长的和的最大值等于____________. 答案：解析：设同一点出发的三条棱长分别为a、b、c,则有a2+b2+c2=l2且对角线在相邻三个面上的射影长分别为、、,∵［(a2+b2)+(b2+c2)+(c2+a2)］(12+12+12)≥()2,即6(a2+b2+c2)≥()2.答案： 完形填空      English people often go out with an umbrella or a raincoat. Why?     1    the weather in Britain often changes quickly.     Spring can be rainy or windy,   2   the weather is getting warmer and you can hope more sunny days. In fact, there   3   as much sunshine in spring as in summer. Summer is   4   time for visitors to go to the seaside. People often go out to have a walk or swim. Autumn is a beautiful season. During autumn it is still nice to be outside, too. In winter, it gets colder, and it might   5    ,especially in the north. There are   6   very heavy winds in this season.     February is usually the coldest   7   of the year, while the warmest   8   often July.     The difference   9     temperature between winter and summer is not so great in Britain. The average temperature for winter is about 4.5℃, and for 10   is about  15.5℃. (     )1. A. Because    (     )2. A. so         (     )3. A. have       (     )4. A. the worst  (     )5. A. rain       (     )6. A. too        (     )7. A. weather    (     )8. A. is    (     )9. A. in    (     )10. A. spring   B. As   B. but  B. can haveB. the bestB. wind  B. eitherB. seasonB. was    B. on      B. summer   C. For        C. or         C. can be     C. the latest C. hot        C. also       C. month      C. are     C. by      C. autumn     D. Since       D. for         D. can         D. the earliestD. snow        D. as well     D. day         D. were        D. for         D. winter