= Ratio-product coordinates
{title2=$u=x/y,\quad v=xy,\quad dx\,dy=(2u)^{-1}du\,dv$}
On the positive quadrant, the inverse coordinate change is $x=\sqrt{uv}$, $y=\sqrt{v/u}$. Its <Jacobian determinant> is $1/(2u)>0$. Regions bounded by positive constant-ratio lines and constant-product hyperbolas become rectangles in these coordinates. An integrand $x/y=u$ then cancels the Jacobian factor, making weighted area integrals particularly simple. The positivity restriction selects a unique square-root branch and fixes orientation.
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