10s 10g lo pc/o 10l p+101g po po C Ln+10/400 =L+△L pc pc p I  2 = p p p 0 2 0 2 0 2 0 2 0 I L ΔL ρc 400 L 10lg ρcI p 10lg p p 10lg /I ρc p 10lg I I L 10lg = +         = +         +         =         =        = LI≈Lp