(2) ∵a?b = 2cos²+ 3sin²= 1 + cos (a+b) +3

∴a = kp +, b = ? kp +, k ÎZ.                                   --- 3分

由a=2 b，得sin() = 0，

∴a = ( 1, sin()), b = (, 3sin()),                       --- 4分

(1) ∵ a+b = ,

         或  P {X³2} = 1 ? ?=» 0.476.                      

18 . (本小题满分14分)

   预言为真的概率P {X³2} = +++= » 0.476.             --- 3分

期望 E (X) = 0´+1´+2´+3´+4´+5´=» 1.67，    --- 3分

          --- 每格1分, 全对8分.