a/ \(\widehat{A_1}+\widehat{A_3}=100^\circ\) mà \(\widehat{A_1}=\widehat{A_3}\)
\(→2\widehat{A_3}=100^\circ↔\widehat{A_3}=50^\circ\)
mà \(\widehat{A_3}+\widehat{A_4}=180^\circ\) (hai góc kề bù)
\(→\widehat{A_4}=180^\circ-\widehat{A_3}=180^\circ-50^\circ=130^\circ\)
b/ \(\widehat{A_1}-\widehat{A_3}=100^\circ\) mà \(\widehat{A_1}=\widehat{A_3}\)
\(→\widehat{A_1}-\widehat{A_1}=100^\circ↔0=0\) (vô lý)
\(→\) Số đo góc \(A_3;A_1\) không xác định
c/ \(2\widehat{A_1}=\widehat{A_4}\) mà \(\widehat{A_1}=\widehat{A_3}\)
\(→2\widehat{A_3}=\widehat{A_4}\\↔2\widehat{A_3}-\widehat{A_4}=0\\↔3\widehat{A_3}-(\widehat{A_3}+\widehat{A_4})=0\)
mà \(\widehat{A_3}+\widehat{A_4}=0\) (hai góc kề bù)
\(→3\widehat{A_3}-180^\circ=0\\↔3\widehat{A_3}=180^\circ\\↔\widehat{A_3}=60^\circ\\→\widehat{A_4}=180^\circ-\widehat{A_3}=180^\circ-60^\circ=120^\circ\)
