a. GT, KL tự ghi
b. Xét \(\Delta ABM,\Delta ACM\) có :
\(\left\{{}\begin{matrix}AB=AC\\AMchung\\MB=MC\end{matrix}\right.\)
\(\Leftrightarrow\Delta ABM=\Delta ACM\left(c-c-c\right)\)
c, \(\Delta ABM=\Delta ACM\left(cmt\right)\)
\(\Leftrightarrow\left\{{}\begin{matrix}\widehat{B}=\widehat{C}\\\widehat{A1}=\widehat{A2}\end{matrix}\right.\)
Mà \(\widehat{B}=40^0\Leftrightarrow\widehat{C}=40^0\)
\(\Leftrightarrow\widehat{BAC}=180^0-40^0.2=100^0\) (định lí tổng 3 góc của tam giác)
\(\widehat{A1}=\widehat{A2}\left(cmt\right)\)
Lại có : \(\widehat{A1}+\widehat{A2}=\widehat{BAC}\)
\(\Leftrightarrow\widehat{A1}=\widehat{A2}=\dfrac{100^0}{2}=50^0\)