CA*CB=2S(ABC)S(ABC)=1/2*AB*h=1/2*(2OC)*(OCsinθ)=OC^2*sinθ2S=2OC^2sinθand 2S=CA*CB=CO^2sosinθ=1/2=>θ=30 ,or 150θ is ∠COAwhen ∠COA=30 ,∠CAB=75when ∠COA=150,∠CAB=15so conclusion is 15