下 * * 个命题：①若|
|
①若|
+a
|=|b
-a
|，则把此等式两边平方，利用向量的数量积的运算性质可得 b
•a
=0，故①正确．b
②若
≠a
，0
•a
=b
•a
，则c
=b
不一定成立，例如当 c
和c
都和b
垂直时，不论它们的模等于多少，都能满足a
•a
=b
•a
，但 c
和c
相等不成立，错误．b
③由|
•a
=b|
|•||a
cos＜b|
，a
＞=|b
|•|a
|，可得cos＜b
，a
＞=1，故 b
和a
是方向相同的共线向量，则b
∥a
成立，正确．b
故只有①③正确，
故答案为①③．
| 完形填空。 | ||||
| When you sit and watch TV hour after hour, do you ever think of what it may be doing to your health? More and more children are becoming over weight (超重)and near-sighted 1 they spend too much time in front of the TV. Also, they are growing less and less creative (有创造性的) since watching TV doesn't 2 any active thought (积极的思维). Last week, the TV-Turnoff Organization（组织）began its ninth TV-Turnoff 3 in the US. The organization encourages people to play games, read books, lie under a tree, talk to family members and friends 4 watching TV. American school children spend, on average, 5 hours a year watching TV-more time than they spend in school. That's too much, say the doctors 6 organized this year's TV-Turnoff Week. Doctors say that the 7 children+ watch television, the more overweight they may become. And the food children eat while they watch the television makes this weight 8 more serious. To keep mentally and physically healthy, children need to take exercise, talk with friends and family, read, and 9 the world around them. Could you go a week without television? It might be 10 for many TV lovers, but why not give it a try? | ||||
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