(2)有阻尼时A=Ae 0.9A=Ae- 0.9 =174s≈3min E (3) ()2 2dt E 0.9=e 2d 2 In 0.9 87≈1.5min 26（2） 有阻尼时 t A A − ' = e 1 0.9 e t A A − = 174s 3min 0.9 ln 1 1 = =   t （3） t A A E E 2 2 ) e ' ( ' − = = 2 2 0.9 e − t = 87s 1.5min 2 0.9 ln 1 2 = =   t