góc A:góc B=3:5; góc B: góc C=1:2

$\frac{\widehat{A}}{\widehat{B}}=\frac{3}{5}\Rightarrow \widehat{A}=\frac{3\widehat{B}}{5}$ ; $\frac{\widehat{B}}{\widehat{C}}=\frac{1}{2}\Rightarrow \widehat{C}=2\widehat{B}$.

$\widehat{A}+\widehat{B}+\widehat{C}={180}^0$(Tổng 3 góc trong tam giác)

$\Rightarrow$ $\frac{3\widehat{B}}{5}+\widehat{B}+2\widehat{B}={180}^0\Rightarrow \widehat{B}={50}^0$$\Rightarrow \widehat{C}={100}^0;\widehat{A}={30}^0$