题目

以原点为圆心，在直线3x+4y+15=0上截得弦长为8的圆的方程是_________;过点A(-3,0)的直线l被圆x2+y2=25所截得的弦长为8，则直线l的方程为_________. 答案：x2+y2=25 x=-3 解析:第一问：设圆的方程为x2+y2=r2,则圆心到直线的距离∴r2=25.第二问：设l:y=k(x+3),当k不存在时，l:x=-3，当x=-3时，y=±4，则弦长=2|y|=8,成立.当k存在时,∴(1+k2)x2+6k2x+9k2-25=0.根据弦长公式：∴k无值. 四． 阅读理解：                          A Professor Reason recently persuaded 35 people to keep a diary of all their absent-minded actions for two weeks. When he came to analyze(分析) their embarrassing errors , he was surprised to find that nearly all of them fell into a few groups .    One of the women, for instance, on leaving her house for work one morning threw her pet dog her ear-rings and tried to fix a dog biscuit on her ear. “ The explanation for this is that the brain is like a computer, ” explains the professor, “ People programme themselves to do certain activities regularly. It was the woman’s custom every morning to throw her dog two biscuits and then put on her ear-rings. But somehow the action got reversed(颠倒) in the programme .” About one in twenty of the incidents the volunteers reported were these “ programme assembly failures.”   Twenty per cent of all errors were “ test failures ”—primarily due to not verifying the progress of what the body was doing . A man about to get his car out of the garage passed through the back yard where his garden jacket and boots were kept , put them on —much to his surprise . A woman victim reported : “ I got into the bath with my socks on .”    The commonest problem was information “ storage failures”. People forgot the names of people whose faces they knew, went into a room and forgot why they were there, mislaid something, or smoked a cigarette without realizing it.    The research so far suggests that while the “ central processor” of the brain is liberated from second-to-second control of a well-practiced routine, it must repeatedly switch back its attention at important decision points to check that the action goes on as intended. Otherwise the activity may be “ captured ” by another frequently and recently used programme, resulting in embarrassing errors. 1. The purpose of Professor Reason’s research is     .     A. to show the difference between men and women in their reasoning     B. to classify and explain some errors in human actions     C. to find the causes which lead to computer failures     D. to compare computer functions with brain workings .  2. Which of the following might be grouped under “ programme assembly failures ”?     A. A woman went into a shop and forgot what to buy.     B. A man returning home after work left his key in the lock.     C. A lady fell as she was concentrating on each step her feet were taking.     D. An old man, with his shoes on, was trying to put on his socks.  3. The word “ verifying ” in paragraph 3 can be replaced by      .      A. improving    B. changing   C. checking  D. stopping  4. According to the passage, the information “ storage failures ” refer to     .     A. the destruction of information collecting system     B. the elimination of one’s total memory     C. the temporary loss of part of one’s memory     D. the separation of one’s action from consciousness