问题 选择题
x1,x2是方x2+x+k=0的两个实根,若恰x12+x1x2+x22=2k2成立,k的值为(  )
A.-1B.
1
2
或-1
C.
1
2
D.-
1
2
或1
答案

根据根与系数的关系,得x1+x2=-1,x1x2=k.

又x12+x1x2+x22=2k2

则(x1+x22-x1x2=2k2

即1-k=2k2

解得k=-1或

1
2

当k=

1
2
时,△=1-2<0,方程没有实数根,应舍去.

∴取k=-1.

故本题选A.

阅读理解

阅读理解    

     When I was at university, I studied very hard. But a Jot of my friends did very little work. Some did

just enough to pass the exams. Others didn't do quite enough. Fred Baines was such a student. He spent

more time drinking in the Tea House than working in the library.    

     Once at the end of the term, we had to take   an important test in Chinese. The test had 100

questions. Beside each question we had to write "True” or “False". While I was studying in my room the

night before the test, Fred was watching TV. Fred usually worried a lot the night before a test. But on that night he looked very easy. Then he told me of his idea.   " 

     It's very simple. There are 100 questions and I have to get 50 right to pass the test. I'll take a coin into

the room. I haven't studied a Chinese book for months, so I'll toss(掷) the coin.  In that way, I'm sure "11 get half of the questions right.”  

     The next day, Fred came happily into the examination room. As he sat tossing a coin for half an hour,

he wrote down his answers. Then he left, half an hour before the rest of us.   

     The next day, Fred met his Chinese teacher at the school gate.  "Oh, good, "he said.  "Have you got

the results of' the test?" The teacher looked at him and smiled, "Ah, it's you, Baines. Just a minute. “Then

the teacher reached his pocket and took out a coin. He threw it into the air, caught in his hand and looked at it,” I’m sorry, Baines, you failed!“  

根据短文内容,判断下列句子的正(T)误(F)。

( )1. Fred Baines was a hard-working student.    

( )2. They had to take a Chinese test.    

( )3. Fred usually worried a lot the night before a test.    

( )4. Fred thought hard and wrote down his answers.    

( )5. Fred passed the test at last.

问答题