当n=1时，显然成立。
假设当n=k时，结论成立，即
则当n=k+1时，
=
=
=
=
matlab验证：
>> D2=[cos(5),-sin(5);sin(5),cos(5)]
D2 =
0.2837 0.9589
-0.9589 0.2837
>>
>>
>> D2*D2
ans =
-0.8391 0.5440
-0.5440 -0.8391
>>
>>
>> D2*D2*D2
ans =
-0.7597 -0.6503
0.6503 -0.7597
>>
>>
>> D3=[cos(15),-sin(15);sin(15),cos(15)]
D3 =
-0.7597 -0.6503
0.6503 -0.7597
>>