＝
＝0．
所以直线MF的倾斜角与直线NF的倾斜角互补，即∠OFA＝∠OFB． 因为OF⊥AB，所以|FA|＝|FB|．……（14分）
19．【解答】解：（Ⅰ）因为a＝1，所以f（x）＝sinx﹣xcosx， 则f'（x）＝xsinx； 当
时，f'（x）≥0恒成立，
所以 f（x）在区间
上单调递增，
所以f（x）≥f（0）＝0；…………（5分） （Ⅱ）因为
，
所以f'（x）＝（a﹣1）cosx+xsinx； ①当a＝1时，由（Ⅰ）知，f（x）≥0对恒成立； ②当a＞1时，因为，所以f'（x）＞0， 因此f（x）在区间上单调递增，
所以f（x）≥f（0）＝0对
恒成立；
③当a＜1时，令g（x）＝f'（x），则g'（x）＝（2﹣a）sinx+xcosx， 因为
，所以g'（x）≥0恒成立，
因此g（x）在区间上单调递增，
且
，
所以存在唯一
使得g（x0）＝0，即f'（x0）＝0；
所以任意x∈（0，x0）时，f'（x）＜0，所以f（x）在（0，x0）上单调递减；所以f（x）＜f（0）＝0，不合题意；……（12分） 综上可知，a的最小值为1．……（13分） 20．【解答】（共13分）
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解：（Ⅰ）：①a11＝4，②数列{a1n}是公差为2的等差数列，③数列{am1}是公比为的等比数列；具有性质A的数阵{aij}3×4，不妨为：
（答案不唯一）．………．（4分）
（Ⅱ）数阵{bij}m×n具有性质A．
只需证明，对于任意的i＝1，2，3，…，n，都有bij＜bi（j+1），其中j＝1，2，3，…，n﹣1．
下面用反证明法证明：
假设存在bpq＞bp（q+1），则b（p+1）q，b（p+2）q，…，bmq都大于bp（q+1），即在第q列中，至少有m﹣p+1个数大于bp（q+1），且bp（q+1）＞b（p﹣1）． （q+1）＞…＞b2（q+1）＞b1（q+1）根据题意，对于每一个bt（q+1）（t＝1，2，…，p），都至少存在一个m}），使得
（it∈{1，2，3，…，
，即在第q列中，至少有p个数小于bp（q+1）．
所以，第q列中至少有m﹣p+1+p＝m+1个数，这与第q列中只有m个数矛盾． 所以假设不成立．
所以数阵{bij}m×n具有性质A．……．（13分）
赠送—初中英语总复习知识点归纳 并列句 and 和，并且， work hard, and you can pass the exam.but 但是 he is rich but he is not happy. Or 否则，要不然，或者（在否定句中表和） Hurry up, or you’ll be late. so 因此，所以 Kate was ill so she didn’t go to school. For 因为 I have to stay up late, for I have a lot of work to do. 状语从句 当状语从句的引导词为If, when, before, after, until, as soon as 等，主句和从句有下列情况： 英语句子中如果一看到 Thought----but----; because----so---这种结构,就是错误.倒装句 so+助动词\\BE动词\\情态动词+另一主语，表示后者与前者一致。so+上句主语+助动词\\BE动词\\情态动词，真的，确实如此。 Tom watched TV last night, so did Ann. Tom didn’t watch TV last night. Neither did Ann. ---You’ve left the light on. ---So I have. I’ll go and turn it off. 宾语从句 ?从句用陈述句语序。?主句与从句的关系。 A．主现从不限；B．主过从过； C．真金不怕火炼。The earth moves around the sun. ③常见的宾语从句。 She says that ---- I hope / think / feel / wonder---- I wonder if he will join us in the discussion tonight. Could you tell / show me--- Could you please tell me where the teacher’s office is? Do you know---- Do you know where Mr. Li lives? Please tell me --- She asked me --- 第页（共15页）
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I don’t know ---- I don’t know whether Tom will go or not.定语从句 that和which在指物的情况下一般都可以互换, 但在下列情况下, 一般用that而不用which。 (1) 先行词为all, everything, nothing, something, anything, little, much 等不定代词时。 I am sure she has something (that) you can borrow. (2)先行词被all, every, no, some, any, little, much等修饰时。I’ve read all the books that are not mine. (3)先行词被序数词或最高级修饰时。This is the first book (that) he has read. (4)先行词被the only, the very, the same, the last修饰时。 This is the very book that belongs to him.一般用that而不用who (1) 先行词是who或who引导的主句。 Who is the girl (that) drove the car? Who (that) broke the window will be punished. (2) 主句以There be 引导时 。 There are 200 people (that) didn’t that和which在指物的情况下一般都可以互换, 但在下列情况下, 一般用which而不用that。 (1)关系代词在限制性定语从句中紧跟介词作宾语（介词提前）。 Those are many trees under (which) they can have a rest. (2) 在非限制性定语从句中。 Football , (which) is a very popular game, is played all over the world. 后跟ing 的词有Finish doing Before 2008 Beijing we will finish building the Olympic Park. enjoy doing 喜欢做某事 I enjoy reading English loudly. mind doing 介意（反对）做某事 would you mind opening the window? practice doing sth. 练习做某事 we should practice speaking English as often as possible. be busy doing sth. 忙于做某事 be worth doing sth. 值得做某事 feel like doing sth =want to do sth. 想要做某事spend --- (in) doing sth 花费时间做某事 stop / prevent / keep ----from doing 阻止某人做某事have trouble \\problem / a hard time doing sth. 做某事很困难 Have fun doing sth. =have a good time doing sth. 做某事很快乐go on doing sth 接着做原来做着的事 go shopping / swimming/ skating /surfing---do some running / washing/ cooking --- 介词(for, with, without, about ---) 后跟动词原形:why don't you why not you'd better (not) would you please (not) make\\ let \\ have\\ [注意: 在被动语态中, t o 要加上]
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