The front view of the cone is a regular triangle, which means that the diameters of the generatrix and the bottom circle of the cone are equal. If the length of the bus is 2, the radius of the bottom circle is 1 and the circumference of the bottom circle is 2π. That is, after the cone is expanded into a sector, the radius of the sector is that the generatrix length of the original cone is 2, and the arc length of the sector is that the circumference of the original bottom circle is 2π. According to the formula of sector arc length: arc length = absolute value of radian number of central angle × sector radius, so 2π = absolute value of radian number of central angle ×2, so radian number of central angle is π, that is, 180 degrees.