已知方程x2-4x+a=0有两个实根，其中一个根小于3，另一个根大于3，则a得取值范围为（）。
A.a≤3
B.a＞3
C.a＜3
D.0＜a＜3
E.以上结论均不正确
参考答案：C
解析：
方法一：根据题意，Δ=(-4)2-4a＞0，得a＜4，不妨设x1＜3，x2＞3，则(x1-3)(x2-3)＜0，即x1x2-3(x1+x2)4+9＜0，从而a-3·4+9＜0，所以a＜3．
方法二：令f(x)=x2-4x+a，如图2.3.1所示，有f(3)＜0即32-4×3+a＜0，解得a＜3．
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| Middle school days are both fun and busy. Students spend their days in class, listening to the interesting and 1 boring words of the teacher. They spend the short breaks running, playing, and shouting happily 2 going back to the classroom again. But 3 should students spend their free time outside school? In Western countries, it is ordinary for students to have a part-time job after school. And at weekends students can make their own money and learn more about the " 4 world". They enjoy the independence (独立) and (of course) money, and parents enjoy the quiet house. 5 , it seems that, in China, parents 6 so much about their children's studies that they would prefer to see their children 7 . most of their free time studying and preparing for all those exams. It is natural for 8 to feel that way in China, but I believe the answer lies in balance (平衡). Sometimes, Western school children work long hours after school to make money, and so they become too tired to listen in class or have no time for homework. But Chinese students spend 9 much time on their studies that all else becomes less important until they want that new MP4 player. So, find a good and healthy balance! If you have a part-time job, you'll be 10 , and your parents will also be happy because you're spending the money you make and not theirs! | ||||
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