已知向量$\overrightarrow{m}$=（$\sqrt{3}$，1），$\overrightarrow{n}$=（0，﹣1），$\overrightarrow{k}$=（t，$\sqrt{3}$），若$\overrightarrow{m}$﹣2$\overrightarrow{n}$与$\overrightarrow{k}$共线，则t=____ ．

如图所示，处于匀强磁场中的两根足够长、电阻不计的平行金属导轨相距1.0m，导轨平面与水平面成θ=37°角，下端连接阻值为R的电阻．匀强磁场方向与导轨平面垂直．质量为0.20kg，电阻不计的金属棒放在两导轨上，棒与导轨垂直并保持良好接触，它们之间的动摩擦因数为0.25．

已知圆C₁：（x﹣2cosθ）²+（y﹣2sinθ）²=1与圆C₂：x²+y²=1，在下列说法中：

①对于任意的θ，圆C₁与圆C₂始终相切；

②对于任意的θ，圆C₁与圆C₂始终有四条公切线；

③当$\theta =\frac{\mathrm{\pi }}{6}$时，圆C₁被直线l:$\sqrt{3}$x-y-1=0截得的弦长为$\sqrt{3}$；

④P，Q分别为圆C₁与圆C₂上的动点，则|PQ|的最大值为4．

其中正确命题的序号为

____ ．

      When one of your car tires goes flat, there are two things you can do, you can complain and change it or, if you don't have a spare, stand helplessly beside the road and hope someone comes to your rescue. Now comes a third choice, called Quick wheel. It is designed to get disabled motorists rolling again as quickly as possible.

      Quick wheel is basically a tiny emergency trailer-complete with three tough little wheels of its own-that supports the flat tire and enables the motorist to drive to a service station without losing much time or expending much energy. The product is made in the Netherlands but is owned and marketed by a US company, Quick wheel Inc. of Greenwich, Connecticut. According to the firm's president, Robert Bockweg, the product meets each of the major worries that customers relate with flat tires: safety, lost time and physical labor.

      To use it, motorists simply unfold the product to its fully extended position, set it in front of the disabled tire, drive the car onto the Quick wheel's ramp(斜板) and fix a special safety strap over the tire. The tire is then locked, or cradled, in Quick wheel's metal frame. Its three wheels do the rest of the work. According to Quick wheel Inc, its product can be driven "for miles" at the speed of up to 45 miles per hour "without any noticeable change in the vehicle's braking (车辆刹车) or steering operation". The company also says that it can be used on just about any type of car, jeep, mini-van or trailer (拖车).

      Bockweg says that Quickwheel will be sold first in the United States, at a price of $150. Sales agreements now being talked over should make the product ready for use in Japan, Canada and Western Europe in the near future. (words: 314 ; time :5ms)