问题 选择题
已知向量
a
=(cos75°,sin75°),
b
=(cos15°,sin15°),则
a
-
b
b
的夹角为(  )
A.30°B.60°C.120°D.150°
答案

∵cos75°=cos(90°-15°)=sin15°,

sin75°=sin(90°-15°)=cos15°

a
=(cos75°,sin75°)=(sin15°,cos15°)

a
-
b
=(sin15°-cos15°,cos15°-sin15°),

∴(

a
-
b
)•
b
=(sin15°-cos15°)cos15°+(cos15°-sin15°)sin15°

=2sin15°cos15°-(cos215°+sin215°)=sin30°-1=-

1
2

又可得|

a
-
b
|=
(sin15°-cos15°)2+(cos15°-sin15°)2

=

2(sin215°+cos215°-2sin15°cos15°)
=
2(1-sin30°)
=1,

|

b
|=
cos215°+sin215°
=1

∴cos<

a
-
b
b
>=
(
a
-
b
)•
b
|
a
-
b
||
b
|
=
-
1
2
1×1
=-
1
2

又∵0°≤<

a
-
b
b
>≤180°,

a
-
b
b
的夹角<
a
-
b
b
>为120°

故选C

选择题
阅读理解

TUESDAY, July11

SHANGHAI---The 12th International Processing, Packaging & End-Line Printing Exhibition opens at Shanghai New International Expo Centre ( to July 13)

Tel: 021 6209 5209

HONGKONG---The Hong Kong International Textile and Garment Machinery and Technology Fair 2006 opens at Hong Kong Convention and Exhibition Centre ( to July 14).

Tel: +852 2584 4333

THURSDAY, July 13

SHANGHAI---The China(Shanghai) International Wedding Photographic Equipment Exhibition (Autumn) opens at Shanghai Mart ( to July16 ).

Tel: 021 6272 2828

HARBIN, Heilongjiang Province---The Ninth Harbin International Automobile Industry Exhibition and Sports Centre ( to July 17 ).

Tel : 0451 5363 5464

FRIDAY, July 14

HONG KONG---The Hong Kong Wedding, Banquet & Wedding Accessories Expo 2006 opens at Hong Kong Convention and Exhibition Centre ( to July 16 ).

Tel: +852 2367 8385

WEDNESDAY, July 19

SHANGHAI---The 14th Shanghai International Advertising Technology & Equipment Exhibition / Shanghai International Media Technology & Equipment opens at Shanghai New International Expo Centre ( to July 22 ).

Tel: 021 6328 8899

SHANGHAI---The Shanghai International Outdoor AD Illuminating Technology & Equipment Exhibition, Outdoor Media Promotion Exhibition opens at Shanghai New International Expo Centre ( to July 22 ).

Tel: 021 6321 7522

SHANGHAI---PAPERTECH Shanghai 2006 opens at Shanghai New International Expo Centre ( to July 22 ).

Tel: 021 6328 8899

小题1:More exhibitions are held in _______

A.Shanghai New International Expo Centre

B.Hong Kong Convention and Exhibition Centre

C.Shanghai Mart

D.Harbin International conference, Exhibition and Sports Centre小题2:How many exhibitions can you attend on July 15______

A.one

B.two

C.three

D.four小题3:The text is likely to be a (an)________

A.business diary

B.product advertisement

C.private timetable

D.magazine