题目

Many years ago, people relied on the sun, the moon and stars to find their way around. Later, the compass was introduced. And now, we have satnav（卫星导航）systems to guide us. A satnav system uses groups of satellites to show the user’s location. They send information to a receiver, such as a smartphone, to show us where we are.The earliest built satnav system is the Global Positioning Satellite System, which belongs to the US. Then there is Russia’s Global Navigation Satellite system，the European Union’s Galileo and China’s own satellite navigation system, Beidou.On October 如图，要把边长为6的正三角形纸板剪去三个三角形，得到正六边形，它的边长为    ．