题188有一种摇奖盘是将一单位圆分成n(n≥3)个均匀的扇形区域构成的(如图1所示),现需将这n个扇形区域用三种不同颜色涂色,并要求三种颜色都要使用,且相邻的区域不能同色.如果把含有n(n≥3)个扇形区域摇奖盘的涂色方法数记为an(图1).图1图2图3(1)求:a3,a4;(2)试用an,an+1表示an+ Problem 188 There is a kind of potter’s wheel that divides a unit circle into n (n ≥ 3) uniform fan-shaped regions (as shown in Figure 1). Now the n fan-shaped regions need to be painted in three different colors. And require three colors to be used, and the adjacent areas can not be the same color. If the number of coloring methods containing n (n ≥ 3) fan-shaped area medallion is recorded as an (Figure 1). Figure 1 Figure 2 Figure 3(1) find: a3, a4; (2) try an, an+1 means an+