| （选修4-2：矩阵与变换） 已知矩阵A=
|
由矩阵A属于特征值6的一个特征向量为α1=
可得1 1 3 3 c d
=61 1
，1 1
即c+d=6； …（3分）
由矩阵A属于特征值1的一个特征向量为α2=
，可得3 -2 3 3 c d
=3 -2
，3 -2
即3c-2d=-2，…（6分）
解得即A=
，…（8分）3 3 2 4
∴A逆矩阵是A-1=
=d ad-bc -b ad-bc -c ad-bc a ad-bc
．2 3 - 1 2 - 1 3 1 2
| 完形填空。 | ||||
| Life gets noisier every day and very 1 people can be free from noise of some kind or another. Whether you live 2 the center of a modern city or a village far away---the chances that you will 3 by planes, cars, radios, etc, are almost everywhere. We seem 4 be used to noise, too. Some people feel quite lonely without background music while they are working. Tests have shown that total silence can be very frightening experiences. 5 , some people enjoy 6 to pop music which is very loud, and this can do harm to their ears. The noise level in some places is far above the usual safety level for heavy industrial areas. One recent report about 7 said that although 8 people say that any noise disturbs their attention, only a sudden change in the level of noise really affects people's attention. It goes on to say that a background noise, which doesn't change too much. Music, for example, may even help people to pay attention. People are testing ways to make 9 noise. There are even laws controlling noise. We cannot return to the "good old days" of peace and quiet. But we can make less noise---if we shout 10 about it. | ||||
|