x*(dy/dx)=lnx-y
y'+1/x*y=(lnx)/x
y' + p(x)•y = q(x)的通解為：
y = [e^-∫ p(x) dx] • [∫ q(x)•[e^∫ p(x) dx] dx + C]
本題
p(x)=1/x
q(x)=lnx/x
∫ p(x) dx=lnx
∫ q(x)•[e^∫ p(x) dx] dx
=∫lnx/x*e^(lnx)dx
=∫lnxdx
=xlnx-∫x*1/xdx
=xlnx-x
故
y = [e^-∫ p(x) dx] • [∫ q(x)•[e^∫ p(x) dx] dx + C]
=e^(-lnx)• (xlnx-x+ C)
=lnx-1+C/x