`~rai~`
\(\text{Xét }\Delta ABC\text{ vuông tại A có:}\\\cos B=\dfrac{AB}{BC}\\\Rightarrow \cos B=\dfrac{3}{6}=\dfrac{1}{2}\\\Rightarrow \widehat{B}=60^\circ\\\text{Ta có:}\widehat{B}+\widehat{C}=90^\circ\quad\text{(2 góc nhọn phụ nhau)}\\\Rightarrow 60^\circ+\widehat{C}=90^\circ\\\Rightarrow \widehat{C}=30^\circ.\\\text{Vậy }\widehat{B}=60^\circ;\widehat{C}=30^\circ.\)