若 $\frac{\sin A}{a}=\frac{\cos B}{b}=\frac{\cos C}{c}$, 则 $\triangle A B C$ 是( )
解析 $\because \frac{\sin A}{a}=\frac{\cos B}{b}, \therefore a \cos B=b \sin A$,

$\therefore 2 R \sin A \cos B=2 R \sin B \sin A, 2 R \sin A \neq 0$.

$\therefore \cos B=\sin B, \therefore B=45^{\circ}$. 同理 $C=45^{\circ}$, 故 $A=90^{\circ}$.