1/A(t)*dA(t)=dln(A(t))
A'(t)=d(A(t))/dt
所以A'(t)/A(t)=[d(A(t))/dt]/A(t)=1/dt*[1/A(t)*dA(t)]=dlnA(t)/dt
写得有点乱,自己整理下应该看得清的