f(X) f(X2) x1)平1-ax) 0x+1=a0x2) f(XI) :::::: X X ax1+(1-a)x2 X2f(X) X αf( x1 ) +(1- α) f( x2 ) f(X1 ) f(X2 ) X1 αx 1+(1 X2 -α)x2 f(αx 1+(1-α)x2 )