2014年6月中国、哈萨克斯坦、吉尔吉斯斯坦跨国申报的“丝绸之路：长安﹣天山廊道的路网”被组织列入“世界文化遗产名录”（    ）

    People use their mouths for many things. In the English language, there are many expressions using the word "mouth". But some of them are not so nice.

    Sometimes, people say something to a friend or a family member. Later they regret because it hurts that person's feelings. Or they tell the person something they didn't mean to tell. The speaker might say: "I really put my foot in my mouth this time."

    Sometimes when one person is speaking, he says the same thing that his friend is going to say. When this happens, the friend might say: "You took the words right out of my mouth!" Sometimes a person has a bad or sad experience with another person. He might say that experience "left a bad taste in my mouth." Or the person possibly has a very frightening(可怕的）experience, like having an angry dog running after him. He might say: "I had my heart in my mouth."

    Some people have lots of money because they were born into a very rich family. You might say the person "was born with a silver spoon (银勺) in his mouth". He is the opposite of a person living from "hand to mouth". This person is very poor and only has enough money for the most important things in life, like food. 

设a，b，l均为不同直线，α，β均为不同平面，给出下列3个命题：

①若α⊥β，a⊂β，则a⊥α；

②若α∥β，a⊂α，b⊂β，则a⊥b可能成立；

③若a⊥l，b⊥l，则a⊥b不可能成立．

其中，正确的个数为（　　）

如图所示，竖直平面内的四分之一圆弧轨道下端与水平桌面相切，小滑块A和B分别静止在圆弧轨道的最高点和最低点．现将A无初速度释放，A与B碰撞后结合为一个整体，并沿桌面滑动．已知圆弧轨道光滑，半径R=0.2m；A和B的质量相等；A和B整体与桌面之间的动摩擦因数μ=0.2．取重力加速度g=10m/s²．求：

超重机的底座、输电线路的支架、自行车的斜支架等，都是采用三角形结构，这样做的数学道理是利用了____ ．