若2a+b=12，其中a≥0，b≥0，又P=3a+2b．试确定P的最小值和最大值．

3D打印的革命性

王飞跃  梁晓辉

    ①3D打印源自100多年前的照相雕塑和地貌成形技术，是一种不需要传统刀具和机床就能打造出任意形状、根据物体的三维模型数据制成实物模型的技术，被认为是一项改变世界的新技术。具体地说，人们首先需要通过电脑对实物进行扫描或直接创建一个三维设计图，再通过3D打印机对这个立体原型进行“切片”，一层一层地进行数据记录，然后将原材料按切片数据层层叠加，直到最终成型。

    ②3D打印机与普通打印机的工作原理基本相同，只是打印材料有些不同，3D打印使用的不是传统的“墨”，而是那些能发生固化反应的材料。比如树脂、塑料、陶瓷、金属等，因而能“打印”出实实在在妁三维立体模型，小到一只耳环，大到体积庞大的工程零件、器具，只要获得了原始的模型数据，3D打印机都能将这些东西精准地复制出来，就像童话中的“复制机”一般神奇。这将使工厂彻底告别车床、钻头、冲压机、制模机等传统工具，从而转变为一种以3D打印为基础的、成本更低、研发周期更短的生产方式。以目前的发展情况判断，3D打印之后，必将是社会制造的迅猛崛起。

    ③据报道，国外目前已经通过3D打印技术成功地打印出了自行车、汽车、电控飞行器等物。现今，3D打印也应用于珠宝制造和模型制作，在时装业、电影业、建筑业等10多个不同的行业和领域显示出了十足的魅力，并大大改变了美国制造业的格局。其实，众多的3D打印创业者此前早已落户以虚拟经济著称的纽约市，即便在2008年金融危机的情况下，3D打印还是拉动了当地的萧条经济，为经济增长贡献了力量。

    ④3D打印的应用并不止于工业制造，它在医学界也有着广阔的发展前景。如果它用来打印的“墨”是一个个活体细胞，那么，只要获得相关器官的切片数据，该器官就可以被“打印”出来。这并非天方夜谭，在去年举行的全球科技娱乐设计大会（TED）上，美国维克森林再生医学学院博士安东尼就向人们展示了这样一个肾脏模型打印的过程。据他介绍，他们正在进行一个更大胆的试验，直接用打印机在人体伤口上进行修复式“打印”如果取得成功，这将是医学领域的重大进展。（选自《人民日报》2012年12月，选文有删改。）

当太阳直射23.5°N纬线时下列说法正确的是（　　）

阅读理解

    There was good news last week for people who struggle to get eight hours of sleep a night: they may not need so much shut-eye after all.

    Researchers from UCLA and elsewhere looked at sleep habits of remote hunter-gatherers in Tanzania, Namibia and Bolivia — groups with pre-industrial lifestyles whose sleep patterns are believed to reflect those of ancient humans. The researchers found that, on average, members of each group sleep a bit less than six and a half hours a night.

    The study, published in the academic journal Current Biology, indicates that “natural” sleep is less than eight hours a night and that modern society's numerous electronic distractions (分心) aren't necessarily to blame for people getting just six or seven hours of sleep.

    “The story that often gets out is that if you sleep for less than seven hours, you're going to die early,” he told me. “That's not true.”

    Yet Americans are addicted to getting more sleep — and on turning to medical shortcuts to help them.

    According to the Centers for Disease Control, as many as 70 million U.S. adults suffer from sleep disorders or sleeplessness. Only a third of Americans get the government's recommended seven to nine hours of sleep a night.

    About 9 million American adults use sleeping pills to help get a good night's rest, the CDC found. Siegel said the number of people relying on medicines “has gone up rather rapidly since then.”

    Industry consulting firm Global Data estimates that worldwide sales for sleeplessness medicines will run about $1.4 billion next year and reach $1.8 billion by 2023, recovering from lower sales in recent years because of cheaper generics(仿制药) hitting the market.

    Dr. Roy Artal, a sleep specialist in West Los Angeles, said it's understandable that busy people in today's go-go world would turn to medicines to speed up the sleep process. But all they're doing is building a reliance on powerful drugs for relatively little improvement.

    “There's no magic pill that makes us sleep when we want and wake up when we want,” Artal said. “The effects of sleeping pills tend to be modest.”

    He and other experts say the answer to sleeplessness isn't to be found in a pill bottle. It's in what's called “sleep hygiene (保健).”