Computing a certain trigonometric determinant

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Hint

Start with $R_2'=R_2-R_1, R_3'=R_3-R_1$

$$\cot B-\cot A=\dfrac{\sin(A-B)}{\sin A\sin B}$$

Using Prove $ \sin(A+B)\sin(A-B)=\sin^2A-\sin^2B $

$$\sin^2B-\sin^2A=-\sin(A+B)\sin(A-B)$$

Finally $A+B=\pi-C$