P5/P0=e^-5k=0.9 -5k=ln0.9 -k=1/5ln0.9 P=P0e^(1/5ln0.9*t) P10/P0=e^(1/5ln0.9*10)=e^(2ln0.9)=e^ln0.81=0.81=81% P/Po=e^(1/5ln0.9*t)=1/2 1/5ln0.9*t=ln0.5 t=5ln0.5/ln0.9=5*0.6931/0.1053=32.89, 33小时
