因为9a^2+9b^2-19c^2=0,所以a^2+b^2=(19/9)c^2tanAtanB/[(tanA+tanB)tanC](上下同时除以tanAtanB)=1/(1/tanA+1/tanB)*1/tanC1/tanA+1/tanB=cosA/sinA+cosB/sinB =(cosAsinB+sinAcosB)/(sinAsinB) =sin(A+B)/(sinAsin...