已知向量a=（2cosφ,2sinφ),φ∈（π/2,π）,向量b=（0,-1）,则向量a与b的夹角为A.3π/2-φ B.π/2+φ C.φ-π/2 D.φ

已知向量a=（2cosφ,2sinφ),φ∈（π/2,π）,向量b=（0,-1）,则向量a与b的夹角为A.3π/2-φ B.π/2+φ C.φ-π/2 D.φ
已知向量a=（2cosφ,2sinφ),φ∈（π/2,π）,向量b=（0,-1）,则向量a与b的夹角为
A.3π/2-φ B.π/2+φ C.φ-π/2 D.φ

已知向量a=（2cosφ,2sinφ),φ∈（π/2,π）,向量b=（0,-1）,则向量a与b的夹角为A.3π/2-φ B.π/2+φ C.φ-π/2 D.φ
【1】可设两个向量a,b的夹角为θ,易知：0º≤θ≤180º.【2】∵向量a=(2cosψ,2sinψ).向量b=(0,-1).∴|a|=2,|b|=1.且a·b=-2sinψ.又cosθ=(a·b)/(|a|×|b|).∴cosθ=(-2sinψ)/2=-sinψ.即有：cosθ=-sinψ.【3】由题设可知：ψ∈(π/2,π),即π/2＜ψ＜π.∴0＜sinψ＜1.∴-1＜-sinψ＜0.∴-1＜cosθ＜0.结合0≤θ≤π,可知：π/2＜θ＜π.【4】∵cosθ=-sinψ.∴cos(π-θ)=sinψ=cos[(π/2)-ψ]=cos[ψ-(π/2)].即有：cos(π-θ)=cos[ψ-(π/2)].由π/2＜θ＜π,且π/2＜ψ＜π可知：0＜π-θ＜π/2.且0＜ψ-（π/2)＜π/2.∴π-θ=ψ-(π/2).∴θ=(3π/2)-ψ.即两个向量a,b的夹角为(3π/2)-ψ.∴选A.

设向量a与b的夹角为θ．
a·b=-2sinφ
|a|·|b|=sqrt[(2cosφ)²+(2sinφ)²]×1=2
∴cosθ=(a·b)/(|a|·|b|)=-sinφ
又∵φ∈(π/2,π)
而θ∈(0,π)
∴θ=3π/2-φ
选A．

Men now affect all aspects of life the Internet, SNS community site to carry out is in full swing. All net, happy network competing market, investment in social networking sites passion, a small amoun...

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Men now affect all aspects of life the Internet, SNS community site to carry out is in full swing. All net, happy network competing market, investment in social networking sites passion, a small amount of financing growth of SNS website from time to time, SNS showed a "feudal disputes" scenes, the industry that, after the SNS community site in a good situation under , hidden deep crisis, the industry reshuffle is inevitable. SNS community site so the face of the situation, how to stand tall talent, is now anxious to deal with industry results. After I investigated a number of SNS contacts and websites, and according to some expert advice and ideas, summed up some tentative ideas, detailed tables are now the following three aspects:
First, take the vertical to carry out the Road: refining market, segment users
Although China's SNS industry to carry out faster, but slower than the four seas, the prevalence of market simulation, learning, and homogenization, which will make a large minority of the Chinese Internet industry is facing serious results, the user will be from the last novel, easy to gradually transformed into product claims to withstand higher, more critical,shoes nike air max, more emotional consumers who seek analysis Ereli analysis flute teacher Cao, "Coupling characteristics of the Chinese market, can make differentiated SNS continued to make good business ability to carry out, SNS also stop in the industry reshuffle. "
Senior members of the Internet, blog, create one of China Fang Xingdong also think that the current international market competition is not enough intense SNS, which will present the homogenization of scene. On innovation, he said, carried out abroad, the vertical type SNS good analogy is worth international own. Carried out on the vertical, is not unfamiliar with the current of everyone, the industry thought, stop vertical conduct has become the launching of the future direction of SNS website.
To carry out the so-called vertical, that is, set the industry vertical sns site, after the needs of the SNS and vertical combination in the common interest of the foundation, the search for enemy fall in love, sharing and discovering their preferences. Disadvantages of vertical sns site is now a secondary table: 1, clearly the purpose of the user; 2, applicability; 3, active high; 4, user loyalty is high; 5, high commercial value.
Vertical mode of operation of the old SNS representatives, human resources, vertical business communities to create a network of people who carry out social networking sites have been made on three concepts: 1, SNS vitality of the basic, to provide users with the ideal life of convergence of the additional value. 2, SNS next step degradation, is the vertical segments and application value. 3, SNS move of the general trend.
From the analysis of the current situation, SNS social networking site has launched the beginning of the road trying to vertical, Men SNS confidence all the entrenched networks and other founding fathers were, and from time to time through the innovative and tireless efforts, finally will be able to Sudden Impact, success is found in my a blue sky.
Second, take the blend, wireless way: the ideal platform for network convergence
SNS social networking sites in the category, for example happy network, for example, Tencent, for example, all networks, should be said that all centers each have their own characteristics and competitive analysis from the current situation, SNS has just started to carry out now is how to deal with the urgent needs of innovative results.
April 12, happy Mr. Guo Wei, vice president of network under the SAN old interview on "innovation" issue, he said, SNS is just the beginning of the launching, the space innovation and development effort is very ambitious. Focus on at most two: one, a blend of a variety of functions; Second, the expansion of the wireless areas.
At present, all international social networking sites, including the happy network, including, in the coming few years doing more of a community concept, but "social" concept has not been fully completed. Men can try the fun may be other community network site, friends to share information (Zhuantie), play the social game, or even a buy (e), and so on, but yet these things are absolutely separate, the future, the direction is to make the combination of the up to Men playing a game, but also share information with friends, complete the booking, listening to music and other things that make social networking more social, idealistic, life. Internet should only be better, to do more fluent in a variety of social functions, to facilitate blending.
Expansion of the wireless areas have great room for growth, burst past two years, the wireless terminal, LBS, etc. used to carry out, SNS social networking sites should be expanded to open up the share of the Internet category, from time to time to expand the strength, talent competition in the future place.
Third, take the road to carry out diversified: combining use with other
SNS community site launched tendency is to stop Web site to carry out diversity, combined with other network industries, for example, buy e-commerce industry, LBS, modest, etc., buy and e-commerce in particular, their website even more into the community with vigor and vitality.
Customers now have to carry more and more industry social networking, middle of last month, Tencent Groupon collaboration with the International Customers network "high friends" formally launched soon, happy network layout is also claimed to buy, after the tens of millions of users to carry out buy the market. Zheng Yujie recently leaked "to the traditional smoking aid buy site user, will need to launch a good product every day. But the social network of users, not to buy as a secondary objective, if not a good product, they will still be logged. That is, speaking, social networking is easier to maintain a large user base. "undoubtedly buy into the industry to carry out the community site has injected new vitality, so that people and the enemy in exchange for hours,nike air max 90, to be quietly shopping loosely.
Another community site can not be ignored in the effort - e-commerce, Commenting on the community network, Mr. Ma has said, "Men carry forward the Men of the disadvantages of e-commerce with the needs of the community together. Men find many international giants companies are exploring the potential of this area. "" To want more from the association of enterprises, we must increase investment in social areas. can not rely on over the move, but the response to carry out restructuring. "Now with the popularity of computers People increasingly rely on networks, e-commerce comes to hot, the national e-commerce transactions in 2007 totaled 2.17 trillion yuan, up 90% over the previous year. China's online shopping carried out rapidly, by the end of June 2008, online shopping to reach 63.29 million subscribers, adding 36.4% within six months. As of December 2008, an e-commerce site users to cover the overall increase was from 90 million to 98 million. It is foreseeable that, e-commerce will be the daily lives of Men will take up a bigger proportion. To west, the combination of e-commerce confidence SNS SNS community site will be launching the limited space.
SNS is a people the basic thrust is based on the ideal of interpersonal relationships as a link on the network as a community expression. SNS also a big disadvantage is the "trust." E-commerce, buy, LBS, and other network industries into thin SNS community site so that people stop trading on emotional communication foundation will become increasingly easy,discount nike air max shoes, even more worrying, and then form a community circle, the circle will happen even effect.

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A
向量a的旋转角（由x轴正半轴逆时针旋转）为φ，b为3π/2