Question 4$AB$ is a chord of a circle with centre $O$. The diameter $CD$ is perpendicular to $AB$, with $AB$ closer to $C$ than to $D$. Given $\angle AOB=90^\circ$, find the quotient $[ABC]/[AOD]$.$\sqrt2-1$$2-\sqrt2$$\sqrt2/2$$(1+\sqrt2)/2$$1/2$