F(y)=P(Y<=y)=P(-2lnx<=y)=P(lnx>=-y/2)=P(x>=e^(-y/2))
=1-P(x<=e^(-y/2))=1-e^(-y/2)
f(y)=F'(y)=(1/2)e^(-y/2)(y∈(0,+∞))