y=(coslnx)^2
y'=2cos(lnx)*[-sin(lnx)]*1/x
=-2/x*sin(lnx)cos(lnx)
=-sin(2lnx)/x
y"=-{[sin(2lnx)]'*x-[sin(2lnx)]*x'}/x^2
=-[x*cos(2lnx)*2/x-sin(2lnx]/x^2
=-[2cos(2lnx)-sin(2lnx]/x^2
所以x=e,y"=-(2cos2-sin2)/e^2