题目

从“充分条件”“必要条件”中选出适当的一种填空：（1）“ax2+bx+c=0(a≠0)有实根”是“ac＜0”的________;(2)“△ABC≌△A′B′C′”是“△ABC∽△A′B′C′”的________. 答案：解析:(1)ax2+bx+c=0(a≠0)有实根b2-4ac≥0b2≥4acac＜0.反之,ac＜0b2-4ac＞0ax2+bx+c=0(a≠0)有实根.所以“ax2+bx+c=0（a≠0)有实根”是“ac＜0”的必要条件.答案：(1)必要条件 (2)充分条件 阅读理解 　　Fear and its companion pain are two of the most useful things that men and animals possess, if they are properly used．If fire did not hurt when it burnt, children would play with it until their hands were burnt away．Similarly, if pain existed but fear didn't, a child would burn himself or herself again and again, because fear would not warn himself or herself to keep away from the fire-that had burnt himself or herself before．A really fearless soldier－and some do exist is not a good soldier because he is soon killed; and a dead soldier is of no use to his army．Fear and pain are therefore two guards without which men and animals might soon die out． 　　In our first sentence we suggested that fear should be properly used．If, for example, you never go out of your house because of the danger of being knocked down and killed in the street by a car, you are letting fear rule you too much．Even in your house you are not absolutely safe：an aero plane may crash on your house, or ants may eat away some of the beams in your roof so that the latter falls on you, or you may get cancer! 　　The important thing is not to let fear rule you, but instead to use fear as your servant and guide．Fear will warn you of dangers; then you have to decide what action to take． 　　In many cases, you can take quick and successful action to avoid the danger．For example, you see a car coming straight towards you．Fear warns you, you jump out of the way, and all is well． 　　In some cases, however, you decide that there is nothing that you can do to avoid the danger．For example, you can't prevent an aero plane crashing onto your house．In this case, fear has given you its warning; you have examined it and decided on your course of action, so fear of this particular danger is no longer of any use to you, and you have to try to overcome it． (1) Children would play with fire until their hands were burnt away if ________． [　　] A． they were given no warning beforehand B． they had never burnt themselves C． they had no sense of pain D． they were fearful of the fire (2) A really fearless soldier ________． [　　] A． is of great use to the army B． is not a real soldier C． is nothing but a dead soldier D． easily gets killed in a battle (3) People sometimes succeed in timely avoiding the danger because ________． [　　] A． they have gained experience B． they jump out of the way in time C． they are calm in the face of danger D． they are warned of the danger by fear and take quick action (4) What's the writer's suggestion when the danger can't be avoided? [　　] A． You have to try to overcome it． B． Fear can really help you to run away． C． Fear always helps you stay safe． D． Fear is of great use to you．