1=10g 101gl Io pc 10k 2+10lg po po pc 400 L,+101g =L.+△L pc LlPc 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