Cho $\alpha,\beta,\gamma \in (0;\dfrac{\pi}{2})$ thoả mãn $\cos\alpha=\tan\beta, \cos\beta=\tan\gamma,\cos\gamma=\tan\alpha$. Chứng minh rằng
$$\sin\alpha=\sin\beta=\sin\gamma=\dfrac{\sqrt{5}-1}{2}$$