Ta có: \(\widehat{A}=2\widehat{C}\) => \(\widehat{C}=\frac{1}{2}\widehat{A}\)
Xét t/giác ABC có: \(\widehat{A}+\widehat{B}+\widehat{C}=180^0\)(tổng 3 góc của 1 t/giác)
=> \(\widehat{A}+\widehat{A}+\frac{1}{2}\widehat{A}=180^0\)
=> \(\frac{5}{2}\widehat{A}=180^0\)
=> \(\widehat{A}=180^0:\frac{5}{2}=72^0\)
+) Theo bài ra ta có :
\(\widehat{A}=2.\widehat{C}\)
\(\Rightarrow\frac{\widehat{A}}{2}=\widehat{C}\)
+) Xét \(\Delta\)ABC có
\(\widehat{A}+\widehat{B}+\widehat{C}=180^o\) ( định lí tổng 3 góc của tam giác )
\(\Rightarrow\widehat{A}+\widehat{A}+\frac{\widehat{A}}{2}=180^o\)
\(\Rightarrow\frac{5.\widehat{A}}{2}=180^o\)
\(\Rightarrow5.\widehat{A}=360^o\)
\(\Rightarrow\) \(\widehat{A}=72^o\)
Vậy \(\widehat{A}=72^o\)
