单选题微分方程y″-7y′+12y=0的通解为______

单选题A salesman makes a profit of 25% on all sales. How many fax machines will he have to sell for $375 each to make a total commission of at least $ 500? (A) 4 (B) 5 (C) 6 (D) 15 (E) 20

单选题某装置的启动密码是由0到9中的3个不同数字组成的，连续3次输入错误密码，就会导致该装置永久关闭，一个仅记得密码是由3个不同数字组成的人能够启动此装置的概率为______．A．B．C．D．E．

单选题x1，x2是方程x2-(k-2)x+(k2+3k+5)=0的两实根，则的取值范围是()．

单选题设f(x)处处可导，则A．B．C．D．

单选题若正方体的对角线之长为4，则正方体的体积为______

单选题曲线y=xex的拐点坐标是 A．(0，1) B．(1，e) C．(-2，-2e-2) D．(-2，-2e2)

单选题下列矩阵中不能相似于对角阵的矩阵是A．B．C．D．

单选题已知，i为虚数单位．若，则a=______．A．B．C．D．

单选题设a＞0，b＞0．若是3a与3b的等比中项，则的最小值为______．A．3B．4C．1D．

单选题Shopping has always been something of an impulse activity, in which objects that catch our fancy while strolling are immediately bought on a whim. Advertisers and sellers have taken advantage of this fact, carefully positioning inexpensive but attractive items on paths that we are most likely to cross, hoping that our human nature will lead to a greater profit for them. With the dawn of the Internet and its exploding use across the world, the same tactics apply. Advertisers now place "banners", links to commercial web sites decorated with attractive pictures designed to catch our eyes while browsing the webs, on key web sites with heavy traffic. They pay top dollar for the right, thus creating profits for the hosting web site as well. These actions are performed in the hopes that during the course of our casual and leisurely web surfing, we'll click on that banner that sparks our interest and thus, in theory, buy the products advertised. Initial results have been positive. Web sites report a huge inflow of cash, both from the advertisers who tempt customers in with the banners and the hosting web sites, which are paid for allowing the banners to be put in place. As trust and confidence in Internet buying increases and information security is heightened with new technology, the volume of buying is increasing, leading to even greater profits. The current situation, however, is not quite as optimistic. Just as magazine readers tend to unconsciously ignore advertisements in their favorite periodicals, web browsers are beginning to allow banners to slip their notice as well. Internet users respond to the flood of banners by viewing them as annoyances, a negative image that is hurting sales, since users are now less reluctant to click on those banners, preferring not to support the system that puts them in place. If Internet advertising is to continue to be a viable and profitable business practice, new methods will need to be considered to reinvigorate the industry. With the recent depression in the technology sector and slowing economy, even new practices may not do the trick. As consumers are saving more and frequenting traditional real estate businesses over their Internet counterparts, the fate of Internet business is called into question. The coming years will be the only reliable indication of whether shopping on the World Wide Web is the wave of the future or simply an impulse activity whose whim has passed. (404 words)Notes: on a whim 心血来潮。surf v.冲浪。in theory在理论上，顺理成章。hosting 访问率离的。call...into question 质疑，对……提出疑问。

单选题甲、乙、丙三人独立地去破译一密码，则该密码被破译的概率为(1)甲、乙、丙能破译出密码的概率分别为(2)甲、乙、丙能破译密码的概率分别为

单选题绝对值不等式|3x-12|≤9的解为______