GMm/(R+h)^2=ma
GMm/R^2=mg
两式相比得
[(R+h)/R]^2=g/a
代入g=9.8m/s^2,a=(5/8)m/s^2,R=6.4*10^6m可求出h
h=3R=3*6.4*10^6m=1.92*10^7m