64. As shown in the figure, points $A, B$ are two points on the line $y=x$. Parallel lines to the $y$-axis are drawn through points $A, B$, intersecting the hyperbola $y=\frac{1}{x}(x>0)$ at points $C, D$. If $BD=2AC$, then the value of $4OC^2-OD^2$ is
answer: 6