$\Delta ABC=\Delta D\text{EF}\text{.}$Ta sẽ suy ra được các cặp cạnh tương ứng bằng nhau

$\widehat{E}=\widehat{B}\Rightarrow \widehat{A}=3\widehat{E}=3\widehat{B}$

$\widehat{C}=\widehat{F}\Rightarrow \widehat{B}=2\widehat{F}=2\widehat{C}$

Mặt khác trong tam giác ABC ta có:

$\widehat{A}+\widehat{B}+\widehat{C}={{180}^{0}}$

$3\widehat{B}+\widehat{B}+\frac{1}{2}\widehat{B}={{180}^{0}}\Rightarrow \widehat{B}={{40}^{0}}$

Suy ra $\widehat{A}=3\widehat{B}={{3.40}^{0}}={{120}^{0}}$