般 y =axu2 +bx 1/2 ■几率 +|b P(+)=P(个)=P(1/2)=|al1 P(-)=P()=P(-1/2)=|b一般 ◼ 几率  = a1/ 2 + b −1/ 2 2 2 2 2 ( ) ( ) ( 1/ 2) ( ) ( ) (1/ 2) 1 P P P b P P P a a b − =  = − = + =  = = + =