1LP min L I()=16+1.cos2πam I6)=1+cos2x）=1,+Mcs201LPIa Imin Imax I0 L I I(x) I I x = 0 + a cos 2  ( ) (1 cos 2 ) (1 cos 2 ) 0 0 0 x I M x II I x I a = +   = +  