| 下面四个命题： ①若x=
②-
③若x，y∈R+，则
④
A、①②B、②③C、②④D、③④ |
对于①，若x=
，则x=0，y=-1时，x+y=-1，故①不成立．1-y2
对于②，由于x=0，
=0；x＞0 时，x x2+1
=x x2+1
≤1 x+ 1 x
； 1 2
x＜0时，
=-1 x+ 1 x
≥-1 -x+ 1 -x
，故有 -1 2
≤1 2
≤x x2+1
，故②正确．1 2
对于③，令x=2，y=3，可得
=x+y 1+x+y
，而 5 6
+x 1+x
=y 1+y
+2 3
=3 4
，故③不正确．17 12
对于④，
+1 1×3
+…+1 2×4
=1 n(n+2)
（1-1 2
+1 3
-1 2
+1 4
-1 3
+…+1 4
-1 n
）=1 n+2
（1+1 2
-1 2
-1 n+1
）1 n+2
＜
（1+1 2
）=1 2
，故④正确．3 4
故选：C．
| 完形填空。 | ||||
| Many people of my generation say that there is no hope for the future because of the way that young people behave today. Their first argument is that when we were 1 we used to look after the older people in our community and help them. They also say that young people today don't care about anything or anyone. 2 , I think the reason why we looked after older people was that we had no 3 . People had to live with their parents and grandparents because they had no money. Young people today earn more and have more freedom to live where they want. 4 this, I think that they are still interested in older people. For example, young people often 5 to help me when I get on and off the bus with heavy shopping. Their second argument is that in our day we didn't 6 to be given jobs-and that young people now don't look for jobs, but just complain about unemployment. On the other hand, things were easier in the past and it was always easy to get a job 7 you had friends and contacts. It is really harder today. Young people complain about unemployment and I think they have 8 reason to complain. In conclusion I think there is 9 for the future. This generation, like generations before them, has new 10 as well as old problems. If they learn from our mistakes the world will be a better place in future. | ||||
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