般情况:f(x) dn dw Ndx dx 分布曲线 f( 曲线下窄条面积 dN △S=f(x)·dx= =dw 0xx+dxi x f(x 曲线下总面积 dw o f(r)dx dw=1分布曲线 L f(x) o x x x + dx 曲线下窄条面积 W N N S f x x d d = ( )d = = x W N x N f x d d d d 一般情况: ( ) = = 曲线下总面积 = = = L L L x W x W f x x 0 0 0 d d 1 d d ( )d L f(x) o x