秦朝专制主义中央集权制度的核心是

已知ABCD为矩形，AB=3，BC=2，在矩形ABCD内随机取一点P，点P到矩形四个顶点的距离都大于1的概率为____．

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    The following are the top things to do in Tampa Bay on the weekend.

    Santa and Ice Skating in St. Petersburg: After visiting Santa, go skating on a simulated (模拟的) ice place. 6:00 pm, North Straub Park, Fifth Avenue NE and Bayshore Drive, St. Petersburg. Free; $5 skating.(727) 893-7441.
    Christmas in the Wild: The animals come out at night for the zoo's new holiday party with carolers (唱圣歌者), performers, a Christmas tree, holiday foods, music and chances with Santa. 9:30 am, Lowry Park Zoo, 1101 W Sligh Ave., Tampa. $ 27. 95 seniors, $22. 95 aged 3—11, 2 and younger free. (813) 935-8552.
    Rudolph the Red Nosed Reindeer: The use of puppets (木偶) improves the attraction of this live version of the 1964 television classic. 3:00 pm, Ruth Eckerd Hall, 1111 N McMullen-Booth Road, Clearwater. $35—$ 100. (727) 791-7400.
    An American in Paris: The Tony Award-winning musical is about an American soldier, a secret French girl and a European city in the consequence of war. Through Dec. 25. 7:30 pm, Carol Morsani Hall at the Straz Center for the Performing Arts, 1010 N MacInnes Place, Tampa. $ 39— $ 90. (813) 229-7827.
    It's A Wonderful Life: A Live Radio Play: An ensemble (重唱) brings a few dozen characters to life with live, on-stage sound effects, to present the story of George Bailey, who considers taking his own life on Christmas Eve. 8:00 pm, American Stage Theatre Company at Raymond James Theater, 163 Third St. N, St. Petersburg. $ 30. (727) 823-7529.

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    See a cell phone cover that you like on Taobao? Forget about placing an order, paying the bill online and waiting for days for it to be delivered to you. In the near future, you'll be able to get it in minutes just by hitting "print" on your computer. You might find it hard to believe that you could actually "print" an object as you would a picture. But it is not that hard to understand how it would work. Just as a traditional printer sprays (喷) ink onto paper line by line, modem 3-D printers spread material onto a surface layer by layer, from the bottom to the top, gradually building up a shape.

    Instead of ink, the materials the 3-D printer uses are mainly plastic, resin (树脂) and certain metals. The thinner each layer is — from a millimeter to less than the width of a hair —the smoother and finer the object will be. This may sound like a completely new technology, but the truth is that 3-D printing has been around since the late 1980s. Back then, it was barely affordable for most people, so few knew about it.

    Last year, though, saw a big change in the 3-D printing industry — printers became much cheaper. For example, 10 years ago a desktop 3-D printer might have cost £20,000, while now they cost only about £ 1,000, according to the BBC. Taken out of the factory and introduced to more diverse and common uses, 3-D printing can create just about anything you can think of --- flutes (笛子), bikinis, jewelry, aircraft parts and even human organs. In fact, scientists from Cornell University in New York have just made an artificial ear using a 3-D printer, according to Science Daily. The fake ear looks and acts exactly like a natural one.

    However, as 3-D printing becomes more common, it may bring about certain problems —such as piracy. "Once you can download a coffee maker, or print out a new set of kitchen utensils (餐具) on your personal 3-D printer, who will visit a retail (零售的) store again?", an expert in 3-D printing told Forbes News. Even more frightening, what if anyone in the world could use a 3-D printer to print out a fully functioning gun?

计算（a+2+ $\frac{1}{a}$ ）÷（a﹣ $\frac{1}{a}$ ）．