dy/dx＝1/lnx * 1/x＝1/(xlnx)
d2y/dx2＝-1/(xlnx)^2 * (lnx+x*1/x)＝-1/(xlnx)^2 * (lnx+1)＝- (lnx+1)/(xlnx)^2
代入x＝e^2得 [(d2y/dx2)|x=e^2]＝-3/(2e^2)^2＝-3/4 *e^(-4)