∴BC*AB=6
Can be seen from the chart
Let b (OA, AB), d (OA/3, AB/3) and c (OA-BC, AB).
Bring points c and d into hyperbola.
(OA-BC)*AB=K →AB*OA-AB*BC=K ②
OA*AB/9=K →AB*OA=9K ③
From ① ② ③
K=3/4