x=1/4*y^2-0.5*lny
f`(y)=0.5*y-1/(2*y)=(y^2-1)/(2*y)
in=(1+(y^2-1)/(2*y))^2)^0.5dy=in(y^2+1)/(2*y)=y^2/4|+0.5lny|=0.75+0.5ln2